... which is equivalent to gimp_parallel_run_async_independent(),
except that it takes an additional "priority" parameter, which
specifies the task's priority, with 0 being the default priority,
and lower values indicating higher priority.  Unlike
gimp_parallel_run_async_full(), the priority parameter doesn't
directly control the task's priority in a queue, but rather, we use
it to control the priority of the task's dedicated thread, on
supported platforms (previously, all independent async tasks would
run with low priority.)

Use low priority when loading fonts, which can take a long time, to
keep the existing behavior.