Don't unnecessarily clear windows with no exposure mask set

Don't unnecessarily clear windows with no exposure mask set

When we just invalidate some area from the app we don't need to clear
windows with no exposure mask, because that wouldn't have happened pre-csw
anyway. Additionally we can avoid such clearing for native windows in cases
where the xserver already did the clearing like on exposes or when resizing
toplevels.

This means we don't fully redraw a GtkSocket when it resizes, thus
avoiding flicker in gnome-mplayer as reported in this bug:
https://bugzilla.gnome.org/show_bug.cgi?id=598050