Overview: dim the background with the dim-factor property

Overview: dim the background with the dim-factor property

Doing this rather than overdrawing a black rectangle saves us
(pixels in screen) * 8 bytes of memory bandwidth for every frame we draw going
into the overview.

It also allows us to dim the background on non-primary monitors making the
overall overview appearance consistent across all monitors.

https://bugzilla.gnome.org/show_bug.cgi?id=656433