Zoom image by `scale`. `row_pct`,`col_pct` select focal point of zoom.

def _zoom(scale:uniform=1.0, row_pct:uniform=0.5, col_pct:uniform=0.5):
    "Zoom image by `scale`. `row_pct`,`col_pct` select focal point of zoom."
    s = 1-1/scale
    col_c = s * (2*col_pct - 1)
    row_c = s * (2*row_pct - 1)
    return _get_zoom_mat(1/scale, 1/scale, col_c, row_c)