`sw`,`sh` scale width,height - `c`,`r` focus col,row.

def _get_zoom_mat(sw:float, sh:float, c:float, r:float)->AffineMatrix:
    "`sw`,`sh` scale width,height - `c`,`r` focus col,row."
    return [[sw, 0,  c],
            [0, sh,  r],
            [0,  0, 1.]]