Update GL/gl.h to match Mesa.
This just copies the ToT version of Mesa's GL/gl.h header into libglvnd. This should allow any applications which specifically depend on Mesa's headers to build against libglvnd instead.
The #ifdefs around the GLeglImageOES typedef should be enough to avoid any duplicate typedef errors in libglvnd just like in Mesa.
You could still run into duplicate typedef problems if you try to include both GL and GLES headers in the same file, but nothing in libglvnd itself does that, and anything else would have had the same problem building against Mesa's headers.
Fixes #194 (closed).