wayland: Drop cairo surfaces when withdrawing

wayland: Drop cairo surfaces when withdrawing

If a surface is unmapped by the client while gdk is processing updates,
(for example Firefox un-mapping its window on Expose events), the
windowing backend resources might be lost (for example with Wayland)
which can cause a crash in end_paint().

Make sure we drop the cairo surfaces as well when hiding the surface,
that will avoid the crash in gdk_surface_impl_wayland_end_paint() when
trying to attach the staging cairo surface to a released wl_surface,
these will be recreated when needed when the surface becomes visible
again and there is no need to keep such buffers around for a surface
which is not visible anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=793062