When a subclass of GtkEntry (e.g. GtkSpinButton) resizes the available
text area (by overriding the get_text_area_size vfunc), we need to
ensure we don't draw a possible progressbar over the part that got
removed from the text area.
This fixes drawing a progressbar in GtkSpinButton and in its subclasses,
such as GimpSpinScale, and makes Mitch happy too!