libepoxy does the opengl extension handling for us.

It also is helpful for trouble-shooting as it prints nice error messages
instead of silently failing or segfaulting in case we do something
wrong, like using gl commands not supported by the current context.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>