gimp:buffer-source-validate is a drop-in replacement for
gegl:buffer-source, however, if the attached buffer has a
validating tile-handler, it makes sure the required region is
validated during process().  This avoids a situation in which
validation happens in different worker threads at the same time
during the processing of a succeeding operation; since validation
is protected by the buffer's tile-storage mutex, this can result in
either a deadlock (currently), or an effective fallback to single-
threaded processing.