Possible freeze break: Mutter and gnome-terminal tranparency

[Owen Taylor <otaylor redhat com>]

OK, so released 2.91.92 releases of GNOME Shell and Mutter today and am
considering things code frozen. But we still have a couple of issues out
that are likely be good to fix. One of them is that we are currently
breaking gnome-terminal transparency by putting a big black opaque
shadow beind transparent terminals.

https://bugzilla.gnome.org/show_bug.cgi?id=635268

The current state for transparent terminals looks like:

 http://fishsoup.net/images/gnome-terminal-broken-transparency.png

I put patches for Mutter in the bug today that give:

 http://fishsoup.net/images/gnome-terminal-fixed-transparency.png

The patches are fairly substantial, but I also consider them reasonably
safe - they don't affect any of the twisty fragile code paths in Mutter
but rather form a code path that is hit reliably every time a
terminal is drawn.

If nothing shows up in review and people think it's appropriate, I'd
like to land these patches before 3.0. Christian may appreciate not
having a pile of gnome-terminal complaining about broken transparency as
well. -)

- Owen