x11: Deal better with L-shaped monitor arrangements
x11: Deal better with L-shaped monitor arrangements

The code for calculating the per-monitor workarea was ignoring
the fact that the EWMH workarea property can only handle rectangular
workareas, and thus can't really do justice to general monitor
arrangements. As a workaround, we ignore it for anything but
the primary monitor. And we ignore it for the primary monitor
as well if it does not even cover it.

https://bugzilla.gnome.org/show_bug.cgi?id=672163

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