work around bug in QThreadPool::tryStart(std::function)

the underlying code doesn't correctly set up ref counting before passing
the internally created runnable to QThreadPoolPrivate. this later
results in failed assertions about the ref counting.

instead create a qrunnable of our function so we can use
tryStart(QRunnable) which has correct ref counting code.

this prevents crashing when used with full on debug builds of Qt

BUG: 449688