let it not be reduced again

[kohunglee]

If the magnification of the image is less than 1 after calculation, or the image in the web page is larger than its own size, let it not be reduced again, so as to prevent the strange effect that the image is not "zoom" but reduced after clicking
as this： this._imgScaleFactor = (this._imgScaleFactor<1) ? 1 : this._imgScaleFactor