Remove slow and unnecessary X11 code-path when using OpenGL functionality

This PR fixes a performance issue on X11 systems making use of the gdk_cairo_draw_from_gl() function in regards to drawing on an OpenGL surface in GTK/GDK. Specifically, this fix removes the slow X11 codepath that was used when executing the gdk_cairo_draw_from_gl() function because it was completely uneeded. I tested the before and after on Flutter and GTK apps making use of the OpenGL rendering and can confirm that there is no behaviour and visual difference. This change is also specific to drawings that make use of textures. The render buffer code path has been left completely unmodified.

Work torwards #4704

@matthiasc Pinging you since you were actively involved in the discussion of that thread.