Add the latest libepoxy.
Revision 6eb075c70e2f91a9c45a90677bd46e8fb0432655.

This includes patches to bring up Android support on libepoxy that aren't yet merged in to libepoxy.
This also includes patches to bring up OS X support without GLX. Patches aren't yet merged upstream.
Change Dolphin over to use libepoxy.

This drops Dolphin's usage of our own GLExtensions backend for getting OpenGL extensions.
Removes GLExtensions
Removes our externals GL.h headers
Adds eglplatform.h and khrplatform.h headers that are required for libepoxy