Calc size of `img` to fit in `crop_target` - adjust based on `do_crop`.

def _get_resize_target(img, crop_target, do_crop=False)->TensorImageSize:
    "Calc size of `img` to fit in `crop_target` - adjust based on `do_crop`."
    if crop_target is None: return None
    ch,r,c = img.shape
    target_r,target_c = crop_target
    ratio = (min if do_crop else max)(r/target_r, c/target_c)
    return ch,int(round(r/ratio)),int(round(c/ratio))