Issue #11412: Prevent selection from affecting NDE filter redraw

This is a proof-of-concept patch to resolve #11412 (closed).

gimp_drawable_filter_apply () uses the current selection as a boundary to draw/redraw a filter. When editing a filter, if there's an active selection created after the filter is added, it interferes with the filter's redraw.

This patch adds a new gimp_drawable_filter_apply_with_mask () function, which can be called if we want to use the existing filter's boundaries to draw the new filter. It temporarily replaces the active selection with the filter's mask before we redraw, then restores the active selection afterewards.