movenc: Make sure the RTP hint tracks have nondecreasing DTS

The RTP timestamps can be decreasing for codecs with B-frames. For
these cases, make sure the timestamps in the MP4 file track itself
are nondecreasing, and add an offset to the RTP packet hint instead
to produce the intended RTP timestamp.

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Rename the timestamp parameter to dts in write_hint_packets

This matches its actual intended use better.

Signed-off-by: Martin Storsjö <martin@martin.st>