h264parse doesn't distinguish between interleaved and mixed

gsth264parse.c says:

      else if (sps->frame_mbs_only_flag == 0) {
        imode = GST_VIDEO_INTERLACE_MODE_MIXED;
      }

So essentially this means "if it's interleaved or mixed, make it mixed and don't bother checking".

The H264 headers contain more information and avdec_h264 parses that information. However, avdec_h264 also says "if we have an interlace-mode from the upstream caps, keep it", so all content is detected as mixed.

Ideally h264parse should correctly parse that information from the headers. Until then, it might be worth modifying avdec_h264 to not believe what upstream says about interlace-mode and always override it, as a workaround.