Theora encoding is broken: choppy and pixelated video

[fireattack]

Basically the same bug as #8040 described, the output video is choppy and pixelated.

Summary of the bug:
How to reproduce:

D:\!test>ffmpeg -i a.mp4 -c:v libtheora -q:v 10 -an -t 5 output.ogv -y
ffmpeg version git-2020-08-16-5df9724 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 10.2.1 (GCC) 20200805
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enabl
e-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enab
le-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enabl
e-libsrt --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --
enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-l
ibvmaf --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom
--enable-libgsm --enable-librav1e --enable-libsvtav1 --disable-w32threads --enable-libmfx --enable-ffnvcodec --enable-cu
da-llvm --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenm
pt --enable-amf
  libavutil      56. 58.100 / 56. 58.100
  libavcodec     58.100.100 / 58.100.100
  libavformat    58. 51.100 / 58. 51.100
  libavdevice    58. 11.101 / 58. 11.101
  libavfilter     7. 87.100 /  7. 87.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'a.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41iso4
    creation_time   : 2020-07-17T12:25:52.000000Z
  Duration: 00:00:11.68, start: 0.000000, bitrate: 639 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720, 506 kb/s, 30 fps, 30 tbr, 600 tbn, 60 t
bc (default)
    Metadata:
      creation_time   : 2020-07-17T12:25:52.000000Z
      handler_name    : Vireo Eyes v2.5.3
      encoder         : AVC Coding
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      creation_time   : 2020-07-17T12:25:52.000000Z
      handler_name    : Vireo Ears v2.5.3
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> theora (libtheora))
Press [q] to stop, [?] for help
Output #0, ogv, to 'output.ogv':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41iso4
    encoder         : Lavf58.51.100
    Stream #0:0(und): Video: theora (libtheora), yuv420p, 1280x720, q=2-31, 200 kb/s, 30 fps, 30 tbn, 30 tbc (default)
    Metadata:
      creation_time   : 2020-07-17T12:25:52.000000Z
      handler_name    : Vireo Eyes v2.5.3
      encoder         : Lavc58.100.100 libtheora
      major_brand     : mp42
      minor_version   : 0
      compatible_brands: mp42mp41iso4
frame=  150 fps= 24 q=-0.0 Lsize=    3080kB time=00:00:05.00 bitrate=5046.3kbits/s speed=0.808x
video:3062kB audio:0kB subtitle:0kB other streams:0kB global headers:3kB muxing overhead: 0.579831%

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.