Keep the panel shadow attached to the panel at all times. Even under screen
transformations like the cube plugin.
(LP: #1011120)
Also fixes other panel shadow bugs (LP: #1061338) and (LP: #1061339). Fixes: https://bugs.launchpad.net/bugs/1011120, https://bugs.launchpad.net/bugs/1061338, https://bugs.launchpad.net/bugs/1061339. Approved by Sam Spilsbury.