The plan is to nuke the custom filter chain completely. It's not clear
what will happen to the still needed builtin filters (mostly hardware
deinterlacing and vf_vapoursynth). Most likely we'll replace them with
different filter chain concept (whose main purpose will be providing
builtin things and bridging to libavfilter).
The undocumented "warn" options are there to disable deprecation
warnings when the player inserts filter automatically.
The same will be done to audio filters, at a later point.
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