Improve 3d realism of compiz by adding a depth component to window position

While Xgl/Compiz/AIGLX/Beryl offer some really nice new features, there are several shortcomings and/or inconsistencies with the current default implementation. In particular, when windows are drawn with shadow, an additional component of window position (i.e. depth) is implied, but not correctly implemented. This specification posits that windows and other desktop objects be given an additional attribute, depth, which modifies how the objects are drawn There are several implications here:

1.) When rotating the desktop, for example using the "cube" desktop switcher, the representation should reflect window depth by showing windows at various heights relative to the cube's surface. This functionality is provided in Beryl via the "3D Windows Layer" plugin. A movie of this plugin in action can be found as http://www.youtube.com/watch?v=PpIhoLzDOTY.

3.) When bringing a new window into focus, if it is occluded by another window, a more realistic transition should be used (rather than windows appearing to "snap" through each other). One possibility is that the occluding window increases in transparency while the new windows to be focused decreases in transparency. This transition should be quite rapid - less than 0.25 seconds - and as subtle as possible.

In general, if 3d enhancements are to make interacting with the desktop more similar to interacting with physical objects in the world, desktop objects should do their best (where appropriate) not to violate physical laws.