Fix dts in f4m

[TPolzer]

If I record BBC1 live with

youtube-dl 'http://a.files.bbci.co.uk/media/live/manifesto/audio_video/simulcast/hds/uk/pc/llnw/bbc_one_hd.f4m'

and play the result with mpv, I get quite a few dropped frames (with vaapi playback) (it seems the display driver was to blame here). The reason seems to be problems with the dts. I get a lot of those in the log:

[ffmpeg/demuxer] mpegts: Invalid timestamps stream=1, pts=325395000, dts=325396800, size=4790
[ffmpeg/demuxer] mpegts: Invalid timestamps stream=1, pts=325398600, dts=325400400, size=5048

I understand that fixing those on the fly is not easy, but probably better than to rely on the decoder to cope with "interesting" input (vaapi decoding is not the only one, streamcopy to mkv with ffmpeg doesn't work at all).