placement: Fix a parent geometry bug in `next_to`.

The function returns the geometry of the parent object. As it was used,
it was passed the parent object directly. The means the parent geometry
of the parent object was used instead of the geometry of the parent
object. This worked "fine" as long as the mouse was in the same screen,
but it was just hiding the bug.

lib/awful/placement.lua

@@ -1533,7 +1533,7 @@ function placement.next_to(d, args)
 
             geo.width, geo.height = dgeo.width, dgeo.height
 
-            fit = fit_in_bounding(pos.region.screen, geo, args)
+            fit = fit_in_bounding(pos.region, geo, args)
 
             if fit then break end
         end