Replace gtk_widget_shape_combine_region() with
gtk_widget_shape_combine_mask() and
gtk_widget_input_shape_combine_region() with
gtk_widget_input_shape_combine_mask().
As GdkBitmap is going away, and the region equivalents already exist,
this seems like pretty much the default step to take.
Includes code to fix up the users.
806a7239ae