Per [1] type 3 chunks/RTMP_PACKET_SIZE_MINIMUM always use
the previously sent (delta) timestamp as their _delta_ timestamp,
so we need to inspect whatever was previously sent, rather than just
looking at the previous packet's absolute timestamp. I.e., type 3
chunks are only permissible in this case if the previously encoded
(delta) timestamp equals the current delta timestamp.
[1] https://rtmp.veriskope.com/docs/spec/#53124-type-3
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