This is the same as with libavfilter.
We will need SPIR-V compilation for at least three different things,
like the VC-2 encoder and decoder, AV1 film grain synthesis for
hardware with no support for it, and possibly other codecs.
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.
The issue is that shaderc_result_get_num_errors may sometime
return 0 even when shaderc_result_get_compilation_status returns
a non-zero error code.
Since we use the result from the former, override the status
if it returned 0.
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.