constraints: Pass constrained relative coordinates to window impl

A placement rule placed window positions itself relative to its parent,
thus converting between relative coordinates to absolute coordinates,
then back to relative coordinates implies unwanted restrictions for
example when the absolute coordinate should not be calculated againts
the current parent window position.

Deal with this by keeping track of the relative position all the way
from the constraining engine to the move-resize window implementation.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/705

src/core/constraints.h

@@ -31,6 +31,8 @@ void meta_window_constrain (MetaWindow          *window,
                             MetaMoveResizeFlags  flags,
                             int                  resize_gravity,
                             const MetaRectangle *orig,
-                            MetaRectangle       *new);
+                            MetaRectangle       *new,
+                            int                 *rel_x,
+                            int                 *rel_y);
 
 #endif /* META_CONSTRAINTS_H */