This doesn't modify internal state, so it can be made a const member
function.

This doesn't modify any internal state, so it can be made a const member
function to allow its use in const contexts.

- Avoid a crash in Xenoblade Chronicles 2.

# Conflicts:
#	src/video_core/engines/kepler_memory.cpp
#	src/video_core/engines/maxwell_3d.cpp
#	src/video_core/morton.cpp
#	src/video_core/morton.h
#	src/video_core/renderer_opengl/gl_global_cache.cpp
#	src/video_core/renderer_opengl/gl_global_cache.h
#	src/video_core/renderer_opengl/gl_rasterizer_cache.cpp

- Fixes a bug with Undertale using 0 for a render target.

- This fixes rendering when changing areas in Super Mario Odyssey.

* 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

We already have a function that does what this code was doing, so let's
use that instead.

We don't need to keep calling the same function over and over again in a
loop, especially when the behavior is slightly non-trivial. We can just
keep a reference to the looked up location and do all the checking and
assignments based off it instead.

It removes a mapping previously created with the MapBufferEx ioctl.

Also removed some dead code and added memory map consistency asserts.

Also moved the GPU MemoryManager class to video_core since it makes more sense for it to be there.