The parallel_distribute() family of functions has been migrated to
GEGL. Remove the gimp_parallel_distribute() functions from
gimp-parallel, and replace all uses of these functions with the
corresponding gegl_parallel_distrubte() functions.
Move gimp_brush_core_{subsample,pressurize,solidify}_mask() to a
separate gimpbrushcore-loops.cc file, so that they can be C++-ified
independently from the rest of the code. The next commit
parallelizes them.
