The issue was that FFv1 concat'd a lot of separate files, each with
a license header, inflating the total shader size to over 3000 lines.
As all codec shaders were rewritten for compile-time SPIR-V, this became
unnencessarily restrictive.
Sponsored-by: Sovereign Tech Fund
The code is not currently used by libavutil, its just where our
common Vulkan code is.
Since SPIR-V compilation will be needed by lavc, move it, rather
than having lavc including lavfi.
It's got a much better API that's actually maintained, it eliminates
race conditions, it comes with a pkg-config file by default, and
unfortunately isn't currently packaged by Debian or other large
distributions.
