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.
