[window] Add type checks to get/set_position
[window] Add type checks to get/set_position

I'd forgotten to add g_return_if_fail checks at the top of these
functions.

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