window-group: Don't create a region when we have an unredirected window

We don't do anything with it other than modify an existing region.

https://bugzilla.gnome.org/show_bug.cgi?id=677116
window-group: Skip the unredirected window

We shouldn't need to process it here.

https://bugzilla.gnome.org/show_bug.cgi?id=677116
window-group: Subtract the unredirected window in stage space

meta_window_group_paint tries to carefully figure out which parts of the
scene it can avoid painting. One area it avoids painting is the region
of the screen occupied by an unredirected window (if there's one present).

In subtracting that region, it gets the coordinate spaces confused, and
ends up subtracting the area at the wrong offset.

This commit translates the region to the correct offset before
subtracting.

https://bugzilla.gnome.org/show_bug.cgi?id=677116
Merge branch 'screensaver-fix'. Backported to muffin by margamanterola.
Credits to Michael Wisheu for bringing this to our attention.
Merge pull request #92 from autarkper/upstream-regions-fix

Upstream regions fix, claims to solve linuxmint/Cinnamon#698