block artifacts with ffvp8

[jzern]

Summary of the bug:
N-106738-g3946cb02fc
Towards the end of the attached clip in the lower right corner a streak of uninitialized/zero (green) blocks will be displayed. The clip decodes correctly with -vcodec libvpx (v1.11.0-192-g2858ef9ec).

How to reproduce:

% ffplay /tmp/1312737577-2s.webm 
ffplay version 4.4.1-3 Copyright (c) 2003-2021 the FFmpeg developers
  built with gcc 11 (Debian 11.2.0-13)
  configuration: --prefix=/usr --extra-version=3 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librabbitmq --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-pocketsphinx --enable-librsvg --enable-libmfx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 70.100 / 56. 70.100
  libavcodec     58.134.100 / 58.134.100
  libavformat    58. 76.100 / 58. 76.100
  libavdevice    58. 13.100 / 58. 13.100
  libavfilter     7.110.100 /  7.110.100
  libswscale      5.  9.100 /  5.  9.100
  libswresample   3.  9.100 /  3.  9.100
  libpostproc    55.  9.100 / 55.  9.100
Input #0, matroska,webm, from '/tmp/1312737577-2s.webm':
  Metadata:
    ENCODER         : Lavf59.22.100
  Duration: 00:00:01.85, start: 0.000000, bitrate: 204 kb/s
  Stream #0:0: Video: vp8, yuv420p(tv, bt470bg/unknown/unknown, progressive), 1920x884, SAR 1:1 DAR 480:221, 25 fps, 25 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      DURATION        : 00:00:01.847000000
   1.36 M-V:  0.157 fd=   0 aq=    0KB vq=    0KB sq=    0B f=0/0  

There is a longer version with additional artifacts, but the attached is sufficient to show some of the issue. I'll create further issues if they persist after this one is fixed.

[Carl Eugen Hoyos]

Regression since 7b5ff7d57355dc608f0fd86e3ab32a2fda65e752, not reproducible with -threads 1

[Balling]

Will be fixed in ​https://patchwork.ffmpeg.org/project/ffmpeg/patch/20220828172545.8185-1-michael@niedermayer.cc/