desktop-shell: Don't attempt to re-add the view to panel layer
This change fixes a side-effect of weston_view_move_to_layer helper which would
basically unmap the view because the layer entry list is no longer
visible. So in order to avoid doing that, just do not attempt to invoke
the committed
callback when the surface is already mapped.
Fixes: #956 (closed)
Signed-off-by: Marius Vlad marius.vlad@collabora.com