lavf: switch to AVStream.time_base as the hint for the muxer timebase
Previously, AVStream.codec.time_base was used for that purpose, which was quite confusing for the callers. This change also opens the path for removing AVStream.codec. The change in the lavf-mkv test is due to the native timebase (1/1000) being used instead of the default one (1/90000), so the packets are now sent to the crc muxer in the same order in which they are demuxed (previously some of them got reordered because of inexact timestamp conversion).
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