Unity panel shadow is "faked" by drawing it on the desktop background

Unity paints its own panel shadow independently of the regular window shadows. This is a redundant hack and should be avoided in future.

Until 13.04, this caused several rendering bugs with respect to other Compiz plugins. However most of those rendering bugs (described below) have now been fixed under separate bug IDs. But the shadow is still "faked" by Unity. Hence this bug.

ORIGINAL DESCRIPTION:
As seen in the attached screenshot, the panel shadow in Unity (both in 11.04 and 11.10) is drawn by simply overlaying it on the desktop background. This causes a bit of ugliness when switching desktops (see the thumbnails in the desktop switcher in the screenshot where one can see a dark line at the top of each desktop), or when toggling expo (there is no panel shadow), and is a bit of a "cheap trick" [with all due respect :)].

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: unity 4.16.0-0ubuntu2
ProcVersionSignature: Ubuntu 3.0.0-11.18-generic 3.0.4
Uname: Linux 3.0.0-11-generic x86_64
ApportVersion: 1.23-0ubuntu1
Architecture: amd64
CompizPlugins: [core,bailer,detection,composite,opengl,imgjpeg,copytex,compiztoolbox,decor,resize,resizeinfo,scale,move,imgpng,put,text,grid,shift,annotate,commands,regex,mousepoll,wall,gnomecompat,place,scaleaddon,animation,workarounds,session,expo,ezoom,unityshell]
Date: Sat Sep 24 18:40:17 2011
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
ProcEnviron:
 LANGUAGE=en_GB:en
 PATH=(custom, user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: unity
UpgradeStatus: Upgraded to oneiric on 2011-09-24 (0 days ago)