If Vulkan was enabled but no SPIR-V headers were found, the Vulkan code
in swscale was still compiled, but no backend was exported. Vulkan
frames were still reported as supported by sws_test_hw_format().
Add a swscale_vulkan configure symbol, enabled when Vulkan and either
SPIR-V header are available, and use it for all Vulkan code in swscale
and for the spirv FATE tests.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>