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.
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