compositor: Unmap views moved to layers outside of the scene graph

commit 5fe02dc6 partially resolved the issue in #937 by bringing back
the old band-aid solution for layer moves. However, since the events of
commit bf228370 and following commits we have a problem with
subsurfaces leaving garbage behind when minimized - see #366 which was
probably fixed for a while then became broken again.

When we minimize a view that has subsurfaces, by moving it to a layer
outside of the scene graph, we need to be sure to handle the subsurface
views - which follow their parent's layer instead of having
weston_view_move_to_layer() explicitly called.

Do this by assuming layers with an empty link are not part of the scene
graph and unmapping views when they're moved to these layers. This will
recursively unmap the subsurface views.

We can now remove the band-aid paint node destroy, as the unmap process
will destroy paint nodes as appropriate.

Fixes 5fe02dc6

Fixes #937

Fixes #366

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>