Bug 784226 - Maximum of processing threads hard-coded to 16.

Raise GIMP_MAX_NUM_THREADS to 64, following the changes in GEGL (see
GEGL commits 6d128ac and f26acbb). This is still considered unstable and
to be used at one's own risk (cf. GIMP commit 1f5739de) but at least, it
could allow discovering and fixing bugs.
It would be nice if GEGL_MAX_THREADS could be public so that to not have
to edit this by hand at each change.