Add the latest libepoxy.
Revision 7422de5b4be7b19d789136b3bb5f932de42db27c.

This includes patches to bring up Android and OS X support on libepoxy that aren't yet merged in to libepoxy.
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