xfwm4: Bug with Xinerama

[Ori Bernstein]

I'm using Xfce with Xinerama (actually, mergedfb on the open-source Radeon
drivers with Xorg 7, monitors running at different resolutions.) with revision
19996. On my main monitor, all windows are stuck with their titlebars above the
panel! this, obviously, makes xfce unusable.

Note that this same revision seems to work fine on my laptop. Also, it's not a
configuration issue, as I've tried with different users as well as moving away
my config.