Otherwise we will consistently report that samples are late. In practice this
can end up causing GStreamer to drop many video samples, which is one of the
apparent causes of bug 50733.

At the same time, don't record jitter *before* signalling preroll, as in that
case stream_start will not yet be valid, and we'll end up recording something
near the current tick count, and in this way also incorrectly report that
samples are late.
Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>