compositor: Destroy paint nodes in weston_view_move_to_layer()

Commit f2486c8b removed some helper functions for layer changes, but
the loop to delete stale paint nodes was elided.

We need to delete paint nodes on a layer change to ensure damage is tracked
properly.

Fixes f2486c8b

Fixes #937

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