1. Only output video starting from a keyframe
2. When calculating the timestamp offset to adjust live streams to start
at t=0, use the timestamp of the first tag from which a sample is actually
output, rather than just the first audio/video tag. The test streams in
the referenced GitHub issue start with a video tag whose packet type is
AVC_PACKET_TYPE_SEQUENCE_HEADER (i.e. does not contain a sample) and whose
timestamp is set to 0 (i.e. isn't set). The timestamp is set correctly on
tags that from which a sample is actually output.
Issue: #6111
PiperOrigin-RevId: 256147747
6febc88dce