This more closely resembles the threading model prescribed by the DirectX SDK
documentation. In particular, the page "Flushing Data" recommends that
IPin::BeginFlush() do the following steps in order:

1. take the filter lock
2. signal the streaming thread to complete
3. take the streaming lock, if necessary
Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>