FFMpegDemuxer: add missing subsampling AV_PIX_FMT_YUVJ420P

[dabrain34]

With given hevc sample generated with GStreamer:

gst-launch-1.0 videotestsrc num_buffers=300 pattern=ball ! timeoverlay ! nvh265enc ! filesink location=/tmp/Sample_300.hevc

[dabrain34]

@zlatinski here is a small patch to address the issue with hevc, discussed earlier.

[zlatinski]

Thank you, dabrain34! I've already merged this change.