After commit 447bc18c EGL on X11 broke.

But the handling of the GL context also was quite awkward because it was
unclear who was responsible for ensuring it got reset.

Change that by making gdk_gl_context_clear_current_if_surface() return
the context (with a reference because it might be the last reference) it
had unset, so that after changing EGL properties the code that caused
the clearing can re-make it current.

This moves the responsibility to the actual code that is dealing with
updating properties and frees the outer layers of code from that task.

And that means the X11 EGL code doesn't need to care and the code in the
Wayland backend that did care can be removed.

Related: !7662
Fixes: #6964 on X11