gegl-parallel provides various parallel algorithms.  Currently,
this is limited to the gegl_parallel_distribute() family of
functions, which distribute work across multiple threads, migrated
from GIMP.

The following commits use these functions to replace the various
thread-pools we use to auto-parallelize operations with simpler
code, fixing potential dealocks as a result of nested operation
processing, as in bug #790810, along the way.

Since gegl-parallel is public API, it also eases manual
parallelization of operations, inside and outside of GEGL.