createGlContext
Creates a new GdkGLContext for the GdkSurface.
The context is disconnected from any particular surface or surface. If the creation of the GdkGLContext failed, @error will be set. Before using the returned GdkGLContext, you will need to call method@Gdk.GLContext.make_current or method@Gdk.GLContext.realize.
Return
the newly created GdkGLContext