gdkwindow-wayland: Bail out early if we get a frame callback when destroying our...

gdkwindow-wayland: Bail out early if we get a frame callback when destroying our window

This prevents warnings like

(gtk3-demo:14948): Gdk-CRITICAL **: _gdk_frame_clock_thaw: assertion 'GDK_IS_FRAME_CLOCK (clock)' failed

(gtk3-demo:14948): Gdk-CRITICAL **: gdk_frame_clock_get_timings: assertion 'GDK_IS_FRAME_CLOCK (frame_clock)' failed

We need to do this, as the compositor might have already sent us a frame
event, in-flight, at the same time we destroy our window. In this case, we'll
receive the then-in-flight "done" event, and then warn as we try to look
up the frame clock on a destroyed window.