Alpha test.

We're currently in feature freeze, let alone API freeze, but I'm submitting this now in the hopes of getting early feedback.

The new interface doesn't have documentation; I'd like to add this, but I think there are some open questions that may require rewriting the interface.

Possibly alpha test function should be a compile option (while the ref should remain a parameter). This is unfortunately not very local, though.

Specifying alpha ref as an immediate constant is probably not very efficient, and it's certainly not how it's done for GLSL in wined3d. However, specifying it as a uniform will take a reroll of the vkd3d_shader_parameter API, which currently has no space to fit a whole buffer in its unions, and can be dealt with independently anyway.

The code is implemented in the spirv backend rather than in common vsir, despite the fact that GLSL will need similar handling. This is because it uses the vkd3d_shader_parameter API, and we cannot handle things like VKD3D_SHADER_PARAMETER_TYPE_SPECIALIZATION_CONSTANT in vsir.