scaleAspectFill:The option to scale the content to fill the size of the view. Some portion of the content may be clipped to fill the view’s bounds.
保持宽高比，填满整个view
scaleAspectFit:The option to scale the content to fit the size of the view by maintaining the aspect ratio. Any remaining area of the view’s bounds is transparent.
保持宽高比，最大适配view；如果view的宽高比，大于填充的，按高度适配；
scaleToFill:The option to scale the content to fit the size of itself by changing the aspect ratio of the content if necessary.
拉伸或者压缩宽高，填满view