_NET_WORKAREA and dual head

[Billy Biggs <vektor dumbterm net>]

  The _NET_WORKAREA property loses some of its meaning when dealing with
xinerama.  It only defines a single rectangle for each desktop, so it is
unclear what to set it to if you have a panel in the middle of two
monitors.

  My assumption is that desktop icons can happily end up behind panels
but I have not tested this.  It seems like a flaw though, even if it's
not worth fixing, a behavior should be specified for this case.  I would
suggest that the work area be limited to one head if a panel is put up
in the middle.  However, I was originally interested in using this
property to avoid looking all of the struts in order to know a good work
area, at which point I would just suggest ignoring middle panels and
only calculating based on panels which line up to one of the 'hard'
edges.

  Thoughts?

  -Billy