combo-box: Fix position when transformed
combo-box: Fix position when transformed

The code that made sure a combo-box remains inside the stage boundaries
always assumed that y extends downwards. This broke the code when
transformations were applied (such as with window rotation). Fix this
assumption, and thus fix the bug.

pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx