Generate video_step from stream and work around wrong field duration.

When demux_mpeg_pes uses FFmpeg for H.264 streams, it doesn't set
video_step as it doesn't know it. But FFmpeg provides it deriveable
from time_base. So let's use the derived value as long as it isn't
set explicitly.
Furthermore, demux_mpeg_pes set's BUF_FLAG_FRAME_END whenever it
encounters a H.264 access unit delimiter, which appears between
fields or frames, but it doesn't know, whether it deals with fields
or frames. avcodec_decode_video() on the other hand sets got_picture
even when the decoded data specifies just a field. But it also sets
the flag interlaced_frame. So let's use this flag to halve video_step
in order to show the decoded images just for a field duration.

--HG--
extra : transplant_source : %40Q%E3fO%3C%E7%A0%02%BA%9D%1D%BD%81%F0f-%EAs%87

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