Freeze break: mutter: switching monitors loses workspaces

[Owen Taylor <otaylor redhat com>]

With Mutter 2.91.92 there are two separate bugs that cause your
workspaces to collapse to a single workspace 

 * If the primary monitor (the laptop screen, say) is on the left, then
   there's a logic bug that causes the workspace assignments to be lost.
 
 * If the primary monitor (the laptop screen, say) is on the right, then
   all the workspaces are lost for a more fundamental reason - the
   behavior of Metacity/Mutter for a long time is that if you hot
   plug a new monitor, all your windows stay at the same X/Y positions
   and hop to the other monitor. Since that's now a secondary monitor
   outside your workspaces, Mutter sees your workspaces as empty
   and deletes everything but the first workspace.

https://bugzilla.gnome.org/show_bug.cgi?id=645408 has patches that
address both of these. I like both patches, and think they are correct
and safe, but I'm much more confident in that judgment for the one-line
patch that fixes the case of primary monitor on the left than the bigger
patch that fixes both cases. So, my instinct is that it makes most sense
to apply only the small patch for 3.0.0 and see if we can get sufficient
testing for 3.0.1 for the larger patch.

I'm however willing to listen to people who think that handling both
cases is really important for 3.0.0, especially if they give the patch
some extensive testing. :-)

- Owen