Fixed generating Metal shaders that make use of fragment barycentric coords

[jay3d]

The title says all.

[CLAassistant]

All committers have signed the CLA.

[cdavis5e]

Changes' descriptions should, you know, describe what the change actually does and, more important, why. 😃 It isn't obvious that this change is necessary or even desirable.

Are you seeing fragment shaders with barycentric inputs that have one of the interpolation decorations? What app/game?

Flat and NoPerspective decorations on these should never be seen. Flat cannot be used at all with barycentrics, and NoPerspective is of course handled by BaryCoordNoPerspKHR. Your change will silently drop Centroid and Sample decorations, which will make shaders that use them "work" but cause incorrect rendering with MSAA when these decorations are used.

[cdavis5e]

This looks wrong.

[jay3d]

Thanks for the comment.

Without my changes, when compiling GLSL shaders to SPIR-V which has gl_BaryCoordNoPersp, then using SPIRV-Cross to generate Metal shaders, this error will be displayed:

Flat, Centroid, Sample, NoPerspective decorations are not supported for BaryCoord inputs.

Which is not entirely true because Metal supports Centroid and NoPerspective decorations for BaryCoord input according to this spec:
https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf
at page 103:

A fragment function input declared with the [[barycentric_coord]] attribute can only be
declared with either the center_perspective (default) or center_no_perspective
sampling and interpolation attributes. The barycentric coordinates and per-pixel primitive ID
can be passed as fragment function input in structures organized as shown in these examples:

struct FragmentInput0 {
    uint primitive_id [[primitive_id]];
    // [[center_perspective]] is the default, so it can be omitted.
    float3 barycentric_coord [[barycentric_coord, center_perspective]];
};

struct FragmentInput1 {
    uint primitive_id [[primitive_id]];
    float2 linear_barycentric_coord [[barycentric_coord,
    center_no_perspective]];
};

So there's clearly a bug there; My changes may not be 100% perfect but they work in my use case.
You may take a look further and advice accordingly.
PS: This PR will complement this: KhronosGroup/glslang#3198

[HansKristian-Work]

Is there any forward progress on this PR?

[rcaridade145]

If DecorationNoPerspective is allowed this line https://github.com/KhronosGroup/SPIRV-Cross/blob/main/spirv_msl.cpp#L11981 should not be here.
Not exactly sure where center_perspective would come from.

[jay3d]

Are you seeing fragment shaders with barycentric inputs that have one of the interpolation decorations? What app/game?

My app, and that's enough for me: https://www.youtube.com/watch?v=_omNIB8q4vM

I'm all ears... How do you suggest this could be fixed?