Caveats:
- Block additional data is ignored if the block is laced
and contains multiple samples. Note that this is not
a loss of functionality (SimpleBlock cannot have block
additional data, and lacing was previously completely
unsupported for Block)
- Subrip and ASS samples are dropped if they're in laced
blocks with multiple samples (I don't think this is
valid anyway)
Issue: #3026
PiperOrigin-RevId: 284545197