The overlay looks nicer with the root window pixmap drawn on the
background. It is scaled up to twice the size, with positioning
based on the rule of thirds.
The sideshow animations shown when entering or leaving the
overlay and toggling the extended view were implemented by
Marina Zhurakhinskaya. They replace the old method of having a
black rectangle behind the workspaces that partly covers the
sideshow during transitions.
configure.ac: Add gdk-x11, clutter-x11 and clutter-glx modules.
overlay.js: Add a root window pixmap actor, make sideshow width
definitions more logical, replace the way the sideshow
animates when entering or leaving the overlay.
workspaces.js: Remove the backdrop, add helper functions for the
overlay transitions.
shell-global.[ch]: Add a method that creates an actor displaying
the root window pixmap and returning clones of it.