Per-sample timing carried through from an MP4 source, so it can be
preserved verbatim when re-muxing (rather than re-derived from
presentation-time deltas). This is what lets reordered (B-frame) streams be
re-muxed correctly.
Reports that an H264Source built from an SRT-file timestamp source
covers fewer frames than the underlying container, because the SRT ran
out of usable stanzas partway through. The source is truncated to the last
complete group of pictures (GOP) that a stanza is available for every
frame of, so reordered (B-frame) streams remain fully resolvable.