ScreenShield: blur and desaturate the screenshield background

The background is the same as the normal desktop, so we blur and
desaturate it to clearly show that it's not the normal system state.
To do so, we don't use standard ClutterEffects, to avoid the FBO
indirection. Instead, we take advantage of MetaBackgroundActor support
for GLSL code and paint the shaded background texture directly.

https://bugzilla.gnome.org/show_bug.cgi?id=682536