DirectShow: Don't seek if playback is stopped

If the playback is already stopped, and after that
new position is requested twice, e.g.
stop();
setPosition(0);
setPosition(0);

this causes the playback to start.

Need to postpone setting of new position until play is requested.

Also no need to emit positionChanged when position is not changed.

Fixes: QTBUG-68778
Change-Id: Ib62942ee155d9c8ed2310132dcc6b501dfef1a04
Reviewed-by: Christian Strømme <christian.stromme@qt.io>