txd: do not set the codec timebase.

It is not supposed to be changed from outside of lavc.
Set the stream timebase and average framerate instead.
lavc: improve AVCodecContext.delay doxy
avconv: always reset packet pts after decoding an audio frame

Currently, if a decoder sets AVFrame.pts, we'd send the same timestamp
to it twice, which is wrong.
lavf: do not use the parser duration for video

The parser has no way of knowing video duration, and therefore no video
parsers set it.