compositor: fix background vignette for non-stretched styles

The background vignette currently fits itself to the painted
texture, instead of the monitor.  This causes some very
wrong looking drawing for backgrounds that don't fill the screen.

This commit reworks the vignette shader code to be clearer, more
correct, and parameterized so that it knows how to scale and
position the vignette.

https://bugzilla.gnome.org/show_bug.cgi?id=694393