window: Force placement for first placement rule

Jonas Ådahl requested to merge jadahl/mutter:wip/fix-popup-multi-monitor into master

If we don't force the placement, we enter the constrain machinery with the position (0, 0), meaning we always get the "current work area" setup to correspond to whatever logical monitor was at that position.

Avoid this by doing the same as "meta_window_force_placement()" and set "window->calc_placement" to TRUE while move-resizing, causing the move-resize to first calculate the initial position.

Fixes: #1098 (closed)

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