This intersects the global work area from _NET_WORKAREA with the monitor
viewport.  The monitor viewport falls back to the core display
dimensions where working RandR is missing.  The _NET_WORKAREA query is
now checked for success.  The _NET_WORKAREA extent array is now indexed
by _NET_CURRENT_DESKTOP.  The _NET_WORKAREA atom is now checked for
availability.

Related to #1322.