Function rows 

pub fn rows(
    a: &FxArgs<'_>,
    dst: &mut Dst<'_>,
    f: impl FnMut(&mut [u8], &[u8], i32, i32),
) -> i32

fx_rows: f(dst_row, src_row, x0, y) for every row of the ROI (rows = the pixels x0..x1); polls the cancel flag before every row -> 0 | ERR_CANCEL