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 branch 'cached-gl-context' into gl-core

Conflicts:
	gfx/context/glx_ctx.c
	gfx/context/wgl_ctx.c
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.
Merge branch 'master' into gl-core

Conflicts:
	libretro-test-gl/libretro-test.c
(OSX) Cleanups to BasicOpenGLView
(iOS) Add IOS ifdefs to RAGameView
(iOS) Precompiled header gets turned into regular header - moved
to compat/ and included only once
(OSX/iOS) Make iOS ObjC code more reusable with OSX
(iOS/OSX) Make frontend code more generic
(OSX/iOS) Start renaming ios_ namespace -functions we're going to implement for
OSX as well