widget: shape_combine_mask => shape_cobine_region

widget: shape_combine_mask => shape_cobine_region

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.