In gimp_gegl_apply_cached_operation(), replace the use of
GeglProcessor with GimpChunkIterator, so that we use the same
chunking logic as for rendering projections.  This has the
advantage of better chunk alignment to the tile grid and dynamic
chunk sizing, which improve performance.

Use chunking even when there's no progress indication, since it
generally results in better cache locality.

(cherry picked from commit 4110f7b7)