image: Allow a transition duration of 0ms
image: Allow a transition duration of 0ms

and thus allow people to desactivate transitions.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2666

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