Include epoxy/gl.h from epoxy/glx.h.

This gives us consistency with GL/glx.h's include of GL/gl.h.
Include epoxy/gl.h instead of epoxy/gl_generated.h from egl_generated.h

This gets us the safety check for including GL/gl.h before epoxy/gl.h
Fix loading of EGL 1.1+ entrypoints.
Fix infinite loop in extension detection when the needle is a substring.

We could keep examining the same ptr value over and over.