mpegpsdemux: Send gap events for late streams whenever updating the SCR

mpegpsdemux: Send gap events for late streams whenever updating the SCR

Instead of doing it on each packet and doing it based on the distance to
the previous SCR instead of based on the DTS.

Previously we would send gap events for audio all the time if the SCR
distance was 400ms because the threshold for audio is 300ms and by only
ever updating the position when the SCR updates we would always be 100ms
above the threshold and send needless gap events.

This fixes audio glitches on various files caused by gap events.