[Cogl] [PATCH] Don't include any GL header from the public GL headers

[Neil Roberts]

Now that the master branch has split off for Cogl 2.0 it seems unlikely
that Mutter will be using master for some time so I've gone ahead and
landed the patch¹.

Regards,
- Neil

1. http://git.gnome.org/browse/cogl/commit/?id=ef5680d3fda5df929dbd0b4