Function evaluate_raw_with 

pub fn evaluate_raw_with(
    threads: Threads,
    shader: &dyn PixelShader,
    constants: &Constants,
    textures: &Views,
    inputs: &BTreeMap<(String, u32), Input>,
    grid: Grid,
    targets: &[(u32, u8)],
    invocation: Invocation,
) -> Result<Raw, Error>

evaluate_raw on threads threads: the grid’s rows (of quads, for Invocation::Quads) worked one per thread at a time, each with its own sampler, the pixels and counts gathered in row order: the same Raw on any count. The first failing row’s error, as the serial run’s.