MSL: Fix support for HLSL mesh shader

[andrejnau]

No description provided.

[CLAassistant]

All committers have signed the CLA.

[andrejnau]

HLSL shader:

struct Output {
    float4 pos: SV_Position;
    float3 color: COLOR0;
};

[outputtopology("triangle")]
[numthreads(1, 1, 1)]
void main(out indices uint3 triangles[1], out vertices Output vertices[3]) {
    SetMeshOutputCounts(3, 1);

    triangles[0] = uint3(0, 1, 2);

    vertices[0].pos = float4(-0.5, -0.5, 0.0, 1.0);
    vertices[0].color = float3(1.0, 0.0, 0.0);

    vertices[1].pos = float4(0.5, -0.5, 0.0, 1.0);
    vertices[1].color = float3(0.0, 1.0, 0.0);

    vertices[2].pos = float4(0.0, 0.5, 0.0, 1.0);
    vertices[2].color = float3(0.0, 0.0, 1.0);
}

Compiled using dxc/spirv-dis from VulkanSDK 1.4.304.1.

[HansKristian-Work]

Seems okay to me. Waiting on CI to pass.