Function decode_simple 

pub fn decode_simple(
    data: &[u8],
    row_stride: usize,
    mapping: &[u16],
    bits: u32,
    cols: usize,
    rows: usize,
    out: &mut [u16],
    parallel: bool,
) -> Result<()>

Decode the uncompressed variant: one little-endian 32-bit word per pixel holding three bits-bit indices into mapping (differences), running sums per colour.