Initial work on core GL context support.
Fix incompatibilities with GL core.
Implement WGL core context.
Add path to cache GL context on reinit.

If successful, can avoid libretro GL reset context callback being
called.
Support cached contexts on Win32.

Only tested on Wine, but works.
Add support for bottom-left origin in libretro GL.
More fixes to core context.
Fix RGUI when using bottom-left.
Merge fixes.
Use glXSwapIntervalEXT when available instead.
Use 3.1 in libretro-test-gl when CORE is used.
Fix some context_reset() calls being missed.
Use glsym symbol loader instead of ugly poking.
Win32 fixups.
Use glsym wrapper in libretro-test-gl.
Add cache_context/destroy notification.
Use context destroy callback in libretro-test-gl.
Clean up a bit.
Fix Android build.
Update Win builds to 0.9.9.3.