The previous behaviour was to damage the entire view, which would
recurse into each popup. This patch makes it damage only the popup's
surface, and respect the surface damage given by the client.
This adds listeners to the popup's map and unmap events rather than
doing the damage in the create and destroy functions. To get the popup's
position relative to the view, a new child_impl function get_root_coords
has been introduced, which traverses up the parents.
1059e173f4