core: change initial sync with --delay, video stream switch

core: change initial sync with --delay, video stream switch

Make A/V sync at the start of playback with nonzero --delay behave the
same way as it does when seeking to the beginning later, meaning video
plays from the start and audio is truncated or padded with silence to
match timing. This was already the default behavior in case the
streams in the file started at different times, but not if the
mismatch was due to --delay. Trigger similar audio synchronization
when switching to a new video stream. Previously, switching a video
stream on after playing for some time in audio-only mode was buggy and
caused initial desync equal to the duration of prior audio-only
playback.