placement: Fix the initial position

When using multiple monitors, with different panels setting struts, xfwm4 tries to compute the initial location based on the max area available.

But in doing so, it's using the window position, which obviously is not the definitive location since we are precisely trying to determine this.

As a result, we would compute the initial position wrong, when mapping a window on a secondary monitor.

To avoid this, compute the maximum available area on the monitor we aim at mapping the window initially, instead of the monitor where the window would be before we place it.

Signed-off-by: Olivier Fourdan fourdan@xfce.org Closes: #532 (closed)