This function is called rarely and blocks quite often for a long time.
So don't waste power and let the CPU sleep.

This might also increase the performance as the other cores might be allowed to clock higher.

* core_timing: Use better reference tracking for EventType.

- Moves ownership of the event to the caller, ensuring we don't fire events for destroyed objects.
- Removes need for unique names - we won't be using this for save states anyways.

* kernel: Replace usage of boost::intrusive_ptr with std::shared_ptr for kernel objects.

- See https://github.com/citra-emu/citra/pull/4710 for details.

This has been hardware tested and it seems that NVFlinger will still 
signal even if there are no buffers to present.

This commit uses guest fences on vSync event instead of an articial fake 
fence we had.
It also corrects to keep signaling display events while loading the game 
as the OS is suppose to send buffers to vSync during that time.

In some cases, our callbacks were using s64 as a parameter, and in other
cases, they were using an int, which is inconsistent.

To make all callbacks consistent, we can just use an s64 as the type for
late cycles, given it gets rid of the need to cast internally.

While we're at it, also resolve some signed/unsigned conversions that
were occurring related to the callback registration.

The NVFlinger service is already passed into services that need to
guarantee its lifetime, so the BufferQueue instances will already live
as long as they're needed. Making them std::shared_ptr instances in this
case is unnecessary.

Like the previous changes made to the Display struct, this prepares the
Layer struct for changes to its interface. Given Layer will be given
more invariants in the future, we convert it into a class to better
signify that.

With the display and layer structures relocated to the vi service, we
can begin giving these a proper interface before beginning to properly
support the display types.

This converts the display struct into a class and provides it with the
necessary functions to preserve behavior within the NVFlinger class.

These are more closely related to the vi service as opposed to the
intermediary nvflinger.

This also places them in their relevant subfolder, as future changes to
these will likely result in subclassing to represent various displays
and services, as they're done within the service itself on hardware.

The reasoning for prefixing the display and layer source files is to
avoid potential clashing if two files with the same name are compiled
(e.g. if 'display.cpp/.h' or 'layer.cpp/.h' is added to another service
at any point), which MSVC will actually warn against. This prevents that
case from occurring.

This also presently coverts the std::array introduced within
f45c25aa back to a std::vector to allow
the forward declaration of the Display type. Forward declaring a type
within a std::vector is allowed since the introduction of N4510
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4510.html) by
Zhihao Yuan.

Gets rid of the largest set of mutable global state within the core.
This also paves a way for eliminating usages of GetInstance() on the
System class as a follow-up.

Note that no behavioral changes have been made, and this simply extracts
the functionality into a class. This also has the benefit of making
dependencies on the core timing functionality explicit within the
relevant interfaces.

Places all of the timing-related functionality under the existing Core
namespace to keep things consistent, rather than having the timing
utilities sitting in its own completely separate namespace.

Converts many of the Find* functions to return a std::optional<T> as
opposed to returning the raw return values directly. This allows
removing a few assertions and handles error cases like the service
itself does.

This member function doesn't actually modify instance state, so it can
be marked as a const member function.

This was missed within #2075. Renames the member function to make it
consistent with the rest of the Find* functions.

This member function doesn't actually modify instance state, so it can
be const-qualified.

This more accurately describes what the function is actually attempting
to do (it's not a simple trivial getter).

Opening a display isn't really a thing to warn about. It's an expected
thing, so this can be a debug log. This also alters the string to
indicate the display name better.

Opening "Default" display reads a little nicer compared to Opening
display Default.

Makes non-mutable state more explicit.

The built-in set of displays is fixed, so we can utilize an array
instead of a vector here.

Upon investigating the issue with #1878, I found that games are the ones who handle the vsync event resetting and not us.

* get rid of boost::optional

* Remove optional references

* Use std::reference_wrapper for optional references

* Fix clang format

* Fix clang format part 2

* Adressed feedback

* Fix clang format and MacOS build

When a destructor isn't defaulted into a cpp file, it can cause the use
of forward declarations to seemingly fail to compile for non-obvious
reasons. It also allows inlining of the construction/destruction logic
all over the place where a constructor or destructor is invoked, which
can lead to code bloat. This isn't so much a worry here, given the
services won't be created and destroyed frequently.

The cause of the above mentioned non-obvious errors can be demonstrated
as follows:

------- Demonstrative example, if you know how the described error happens, skip forwards -------

Assume we have the following in the header, which we'll call "thing.h":

\#include <memory>

// Forward declaration. For example purposes, assume the definition
// of Object is in some header named "object.h"
class Object;

class Thing {
public:
    // assume no constructors or destructors are specified here,
    // or the constructors/destructors are defined as:
    //
    // Thing() = default;
    // ~Thing() = default;
    //

    // ... Some interface member functions would be defined here

private:
    std::shared_ptr<Object> obj;
};

If this header is included in a cpp file, (which we'll call "main.cpp"),
this will result in a compilation error, because even though no
destructor is specified, the destructor will still need to be generated by
the compiler because std::shared_ptr's destructor is *not* trivial (in
other words, it does something other than nothing), as std::shared_ptr's
destructor needs to do two things:

1. Decrement the shared reference count of the object being pointed to,
   and if the reference count decrements to zero,

2. Free the Object instance's memory (aka deallocate the memory it's
   pointing to).

And so the compiler generates the code for the destructor doing this inside main.cpp.

Now, keep in mind, the Object forward declaration is not a complete type. All it
does is tell the compiler "a type named Object exists" and allows us to
use the name in certain situations to avoid a header dependency. So the
compiler needs to generate destruction code for Object, but the compiler
doesn't know *how* to destruct it. A forward declaration doesn't tell
the compiler anything about Object's constructor or destructor. So, the
compiler will issue an error in this case because it's undefined
behavior to try and deallocate (or construct) an incomplete type and
std::shared_ptr and std::unique_ptr make sure this isn't the case
internally.

Now, if we had defaulted the destructor in "thing.cpp", where we also
include "object.h", this would never be an issue, as the destructor
would only have its code generated in one place, and it would be in a
place where the full class definition of Object would be visible to the
compiler.

---------------------- End example ----------------------------

Given these service classes are more than certainly going to change in
the future, this defaults the constructors and destructors into the
relevant cpp files to make the construction and destruction of all of
the services consistent and unlikely to run into cases where forward
declarations are indirectly causing compilation errors. It also has the
plus of avoiding the need to rebuild several services if destruction
logic changes, since it would only be necessary to recompile the single
cpp file.