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.
Finally, this is as close to usable as it gets for glslang.
Much faster to compile as well, and eliminates the need for a C++
compiler, which is great.
Also, changes to the resource limits won't break users, as we
can use designated initializers in C90.