This avoids creating a bunch of threads that fills up the maximum amount
allowed by QThreadPool, and causes a deadlock between the helper threads
and the main thread (main thread tries to create threads in painting
code, but isn't able to, so it keeps waiting for a thread to free up,
but all the threads are waiting on the main thread to process some
events).
Signed-off-by: flow <flowlnlnln@gmail.com>
c440f33122