For several reasons.

Firstly, the reference clock should still function when the filter is not
running.

Secondly, IDirectSoundBuffer::GetPositions() in practice returns very coarse
positions, both on Windows and on Wine. On my hardware, the resolution is
about 10ms, which, while suitable for the DirectSound renderer and probably
also any video renderers, is nevertheless actually coarser than
GetTickCount().

Thirdly, testing supports that the native DirectSound renderer returns a
timestamp from IReferenceClock::GetTime() that is more accurate than
IDirectSoundBuffer::GetPositions(). In fact, after dumping a large number of
different clock sources, I came to the conclusion that it is probably using
timeGetTime() as a source. On Wine that's identical to GetTickCount(), so we
may as well just delegate directly to the system clock.
Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>