modal-frame: Use the offscreen clone ability
modal-frame: Use the offscreen clone ability

Instead of relying on a slightly hacky patch to Clutter, use MxOffscreen's
new cloning ability to do the same thing. This now works fine with
stock Clutter 1.2.

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