In the gimp_parallel_run_async() family of functions, allow the
async callback to return without completing the async operation, in
which case the callback will be called again, until the operation
is either completed, or canceled, in which case it is aborted
(previously, returning from the callback without completing the
operation would cause it to be aborted.)  It is guaranteed that all
operations of the same priority will get a chance to run, even if
some of them contuinuosly return without completing.

This allows potentially time-consuming operations to yield
execution in favor of other same-priority operations, and, in
particular, of higher-priority operations, to avoid priority
inversion.  Essentially, this allows a simple form of cooperative
multitasking among async operations.

(cherry picked from commit 4969d757)