Add gdk_window_mark_paint_from_clip and call from widget drawing

This is a new function that gets called every time we're drawing
some area in the Gtk paint machinery. It is a no-op right now, but
it will be required later to keep track of what areas which
we previously rendered with GL was overwritten with cairo contents.

docs/reference/gdk/gdk3-sections.txt

@@ -394,6 +394,7 @@ gdk_window_constrain_size
 gdk_window_beep
 gdk_window_get_scale_factor
 gdk_window_set_opaque_region
+gdk_window_mark_paint_from_clip
 
 <SUBSECTION>
 gdk_window_get_clip_region