Add gimp-parallel.[cc,h], which provides a set of parallel
algorithms.

These currently include:

  - gimp_parallel_distribute():  Calls a callback function in
    parallel on multiple threads, passing it the current thread
    index, and the total number of threads.  Allows specifying the
    maximal number of threads used.

  - gimp_parallel_distribute_range():  Splits a range of integers
    between multiple threads, passing the sub-range to a callback
    function.  Allows specifying the minimal sub-range size.

  - gimp_parallel_distribute_area():  Splits a rectangular area
    between multiple threads, passing the sub-area to a callback
    function.  Allows specifying the minimal sub-area.

The callback function is passed using an appropriately-typed
function pointer, and a user-data pointer.  Additionally, when used
in a C++ file, each of the above functions has an overloaded
template version, taking the callback through a generic parameter,
without a user-data pointer, which allows using function objects.