With all the various refactorings that have happened over the years,
the current pts logic is very broken for non-trivial cases (ie: ones
where not every frame/field has a meaningful pts assocated with it).

Generally, we do not want to write AV_NOPTS_VALUE as the output
timestamp, regardless of anything else. It's better to pass zero
if there's no other information.

Additionally, interlaced content where the decoder returns each field
separately can result in the first field carrying the timestamp and
the second having AV_NOPTS_VALUE. It's clearly wrong to overwrite
the valid timestamp.

So, let's just never write AV_NOPTS_VALUE into an output frame.

Empirically, this fixed playback of interlaced mpeg2 and h.264 and
mpeg4-asp with packed b-frames in an avi container.