libavcodec/mpegvideo_parser: improve detection of progressive mpeg2

libavcodec/mpegvideo_parser: improve detection of progressive mpeg2

Previously many progressive mpeg2video samples were detected as interlaced
by ffmpeg/ffprobe. For example, https://tmm1.s3.amazonaws.com/720p.ts

Before:

    Input #0, mpegts, from 'https://tmm1.s3.amazonaws.com/720p.ts':
      Duration: 00:00:08.62, start: 18974.073233, bitrate: 9734 kb/s
      Program 2
    	Stream #0:0[0x12eb]: Video: mpeg2video (Main), yuv420p(tv, bottom first), 1280x720 ...

After:

    Input #0, mpegts, from 'https://tmm1.s3.amazonaws.com/720p.ts':
      Duration: 00:00:08.62, start: 18974.073233, bitrate: 9734 kb/s
      Program 2
    	Stream #0:0[0x12eb]: Video: mpeg2video (Main), yuv420p(tv, progressive), 1280x720 ...

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>