[pixel shader]
float4 main() : SV_target
{
    float4 ret = float4(2, 2, 2, 2);
    ret.xyzw.xyzw -= float4(0.1, 0.2, 0.3, 0.4);
    return ret;
}

[test]
todo(glsl) draw quad
probe (0, 0) rgba (1.9, 1.8, 1.7, 1.6)