Adds following settings:
<placement>
<policy>cascade</policy>
<cascadeOffset x="40" y="30" />
</placement>
"Cascade" policy places a new window at the center of the screen like
"center" policy, but possibly shifts its position to bottom-right so the
new window doesn't cover existing windows.
The algorithm is copied from KWin's implementation:
df9f8f8346/src/placement.cpp (L589)
Also added some helper functions to manipulate `wlr_box`.
When growing or shrinking a view by snapping to an edge, a client may
ignore the requested size and instead keep its original size or
substitute a different (possibly constrained) size. In this case, the
view may not actually contact the snapped edge, and a subsequent snap
attempt will just keep re-trying (and failing) to contact the same ege.
To mitigate this, remember the last-snapped view, snapping direction and
offset of the snapping edge in snap.c; when re-attempting a snap for the
same view in the same direction, ignore the edge that was last "hit", to
allow snapping to progress beyond the problematic edge.
...to edges actually encountered by motion during interactive moves and
resizes.
In addition, ignore edge resistance and attraction for minimized views.
