Add a boolean "independent" parameter to gimp_parallel_run_async().
When FALSE, the passed function is run in the shared async thread
pool; when TRUE, the passed function is run in an independent
thread.

Generally, async operations should run in the async pool, however,
it might be desirable to run long-standing operations, especially
ones that can't be canceled, in independent threads.  This avoids
stalling quicker operations, and shutdown.

Adapt the rest of the code for the change.  In particular,
initialize the font cache in an independent thread.

(cherry picked from commit ad8add68)