citra: Remove GLFW, Add SDL2

FindSDL2: Do not CACHE SDL2_* variables if library is not found

EmuWindow_SDL2: Set minimal client area at initialisation time

EmuWindow_SDL2: Corrections

EmuWindow_SDL2: Fix no decorations on startup on OS X

cmake: windows_copy_files

This commit:
* Adds a new subproject, audio_core.
* Defines structures that exist in DSP shared memory.
* Hooks up various other parts of the emulator into audio core.

This sets the foundation for a later HLE DSP implementation.

- Also implement D0 LUT enable.

- No functional difference.

- Gets us LUT interpolation for free.
- Some older Intel GPU drivers did not support the big UBOs needed to store the LUTs.

- Diffuse
- Distance attenuation
- float16/float20 types
- Vertex Shader 'view' output