This commit rounds the timestamp to the integer multiple of fps_den.
Prior to this change, since the timestamp in MFX is defined in units of
90 kHz, timestamps for 60000/1001 fps is not accurately expressed. Also
the encoder sometimes returns the decode time stamp slightly off.
Hence, the timestamp in OBS becomes not a multiple of fps_den after
converting back from the timestamp in MFX.
56c502b254