compositor: Flush surface damage to surface below on destroy

compositor: Flush surface damage to surface below on destroy

When we destroy a surface, we damage the surface below so that the area
exposed by the disappearing surface will be repainted.  However, if that
surface also is destroyed, the damage information is lost and we fail to
repaint that area.

This commit introduces weston_surface_flush_damage(), which flushes the
surface damage the the surface below when a surface is destroyed.  When
multiple surfaces are destroyed at the same time, the damage now accumulates
and sinks down through the surface stack as it should.