On some platforms we can ask the GL context machinery to create a GLES
context, instead of a GL one.

In order to ask for a GLES context at GdkGLContext realization time, we
use a bit field like we do for forward compatible, or debug contexts.

The 'use-es' bit also changes the way we select a default version,
because OpenGL and OpenGLES versions differ.

https://bugzilla.gnome.org/show_bug.cgi?id=743746