The QVideoFrame documentation explicitly says that the time is in
microseconds, however the GStreamer backend was setting the time in
milliseconds and the WMF backend in 100-nanosecond units.
With WMF, the time was missing from the QVideoFrame when presenting it to
the video surface.

Task-number: QTBUG-31731
Change-Id: I0638d2abf8eed25b3a531db67c19a18703e5b630
Reviewed-by: Andy Nichols <andy.nichols@digia.com>