vkd3d-shader: EvaluateAttributeSnapped offset broken

[DadSchoorse]

This issue is just a reminder that the intrinsic is broken, nothing really uses it in practice.

DXBC/DXIL's offset is some weird fixed point, SPIR-V's (interpolateAtOffset) is a float. vkd3d-shader doesn't convert it, so it effectively always passes a denorm. dxil-spirv gets it right.