Subgroup quad operations

[JMS55]

Wgpu 0.20 landed subgroup operations, but is missing quad operations.

E.g. Fidelity FX's single pass downscaling uses them:

FfxFloat32x4 SpdReduceQuad(FfxFloat32x4 v) 
{
    FfxFloat32x4 v0 = v;
    FfxFloat32x4 v1 = subgroupQuadSwapHorizontal(v);
    FfxFloat32x4 v2 = subgroupQuadSwapVertical(v);
    FfxFloat32x4 v3 = subgroupQuadSwapDiagonal(v);
    return SpdReduce4(v0, v1, v2, v3);
}