app: some cleanup in gimppaintcore-loops
In gimp_paint_core_loops_process(), initialize the iterator with sufficient room for the number of iterators used by the algorithm hierarchy, instead of a fixed number. Add an additional 'rect' parameter to the init_step() and process_rows() algorithm member functions, which receives the area of the currently-processed chunk, to be used instead of the iterator's ROI member. This allows us to pass a NULL iterator to hierarchies that don't use an iterator, and avoid the stack- allocated iterator hack we used in this case (and which became even more problematic with the new iterator API).
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