Some callbacks may be pushed during pipeline initialization and before
the dispatch thread is actually started and waiting for them, which
sometimes causes some hangs.

This fixes Fallout 3 and Panzer Corps -and possibly others- intro videos
hanging forever.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>