* introduce libavcodec-extra/libavfilter-extra flavor
    + build with configure flag --enable-libglslang
      build-depend on glslang-dev
    + build with configure flag --enable-vulkan
      build-depend on libvulkan-dev
    + build-depend on libamf-dev
      when AMF is in Debian
      closes: bug#966520
  * build with configure flag --enable-avisynth
    when AviSynth+ is in Debian
    (embedded avisynth code dropped upstream)
  * build with configure flag --enable-decklink
    when DeckLink SDK header files are in Debian
    closes: bug#839941
  * build with configure flag --enable-libilbc
    when libilbc is in Debian
    closes: bug#675959
    <https://github.com/TimothyGu/libilbc>
  * build with configure flag --enable-libklvanc
    when libklvanc is in Debian
    <https://github.com/stoth68000/libklvanc>
  * build with configure flag --enable-libkvazaar
    when Kvazaar is in Debian
    <https://github.com/ultravideo/kvazaar>
  * build with configure flag --enable-libopenh264
    when OpenH264 is in Debian
    <https://github.com/cisco/openh264>
  * build with configure flag --enable-librav1e
    when rav1e is in Debian
    closes: bug#920842
    <https://github.com/xiph/rav1e>
  * build with configure flag --enable-libtensorflow
    when Tensorflow is in Debian
    closes: bug#959765
  * build with configure flag --enable-libvmaf
    when VMAF is in Debian
    <https://github.com/Netflix/vmaf>
  * build with configure flag --enable-libxavs
    when XAVS is in Debian
    <http://xavs.sourceforge.net/>
  * build with configure flag --enable-vapoursynth
    when vapoursynth is in Debian
    closes: bug#833942
    <https://github.com/vapoursynth/vapoursynth>
  * build with configure flag --enable-libdavs2
    when davs2 is no longer deemed experimental
  * build with configure flag --enable-libxavs2
    when xavs2 is no longer deemed experimental
  * build with configure flag --enable-liblensfun
    when a released version of liblensfun is supported again