Disable OpenGL when Gtk3 is selected

The OpenGL library is not supported on Gtk3 which uses the cairo
library for rendering. Cairo, in turn uses hardware acceleration when
supported by the underlying system. Some improvement to performance
may be gained by useing the cairo-gl and cairo-glx libraries included
on modern Gtk3 systems.

See the cairo documentation for further information. The following
link may also be of interest.

http://lists.cairographics.org/archives/cairo/2012-October/023609.html