gdk: Increase generalization of gl_context_set_required_version
This way the warning provides better information when the user tries to set a OpenGL ES context of a too low version, and avoids the warning getting out-of-sync with the minimum version required.
Also, avoid using gdk_gl_context_get_use_es, since it will always return FALSE because the context hasn't been realized.
Edited by Pablo Correa Gomez