Change Dolphin over to use libepoxy

This drops Dolphin's usage of our own GLExtensions backend for getting OpenGL extensions and uses libepoxy instead.

This has been tested on:
Android - EGL + GLES3, EGL + GL
Linux - GLX + GL, EGL + GL, EGL + GLES3

This won't currently work on Windows since it isn't compiled with libepoxy support.
It was untested on OS X, no idea if it works there.