Ell authored Sep 11, 2018 and Øyvind "pippin" Kolås committed Sep 18, 2018

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).

(cherry picked from commit 6c6a7514)