If mutter_plugin_set_stage_input_region() and related methods are
called before the output window is available, save the set input
region and apply it to the output window later on.

compository-mutter.[ch]: Add mutter_set_stage_input_region() and
mutter_empty_stage_input_region(), move the input region handling
here and implement the delayed shape setting described
above.

mutter-plugin.c: Implement shape setting in terms of the new methods.