[pixel shader]
float4 main(): sv_target
{
    float4 ret;
    return (ret = float4(0.1, 0.2, 0.3, 0.4)), ret + 0.5;
}

[test]
todo(msl) draw quad
probe (0, 0) rgba (0.6, 0.7, 0.8, 0.9)