Useful for formats where they provide the downmix matrix in the bistream rather than scale factors for channel groups.
$ ./ffmpeg -downmix stereo -i $fate-suite/dts/dcadec-suite/core_51_24_48_768_1.dtshd -y native.wav
Only for primary channel sets if they embeed coefficients for now.
Instead of reimplementing the same code on every afq user, just do it here.
The format stores video-only H.264 in a fixed header + index table + chunk layout.
vf_libplacebo is unique in that it can function as both a Vulkan filter, and a software filter, depending on how it the filter graph is configured.
A format-only conversion leaves the auto use_filters value unresolved.