Implement non-linear overview shade for background

Adding a radial gradent to the dimming effect gives more depth to
the background.
Shading is computed in a GLSL fragment shader, and uses distance to
center of the screen to interpolate the darkening value to use.

Based on a patch by Pierre-Eric Pelloux-Prayer <pelloux@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=669798