什么叫离屏渲染：
当GPU无法直接把渲染结构写入帧缓冲区，而是先创建一个临时的区域，之后再写入帧缓冲区，这个过程叫离屏渲染
什么会触发离屏渲染：
1.光栅
UIImageView*imgView = [[UIImageViewalloc]initWithFrame:CGRectMake(50,50,100,100)];
imgView.image= [UIImageimageNamed:@"logo"];
imgView.layer.shouldRasterize = YES;
[self.viewaddSubview:imgView];
2.遮罩
UIImageView*imgView = [[UIImageViewalloc]initWithFrame:CGRectMake(50,50,100,100)];
imgView.image= [UIImageimageNamed:@"logo"];
[self.viewaddSubview:imgView];
CALayer*layer = [CALayerlayer];
layer.frame=CGRectMake(30,30,100,100);
layer.backgroundColor = [UIColor redColor].CGColor;
imgView.layer.mask= layer;
3.阴影，如果alpha=1则不会造成离屏渲染，可以通过设置shadowPath来避免离屏渲染
UIImageView*imgView = [[UIImageViewalloc]initWithFrame:CGRectMake(50,50,100,100)];
imgView.image= [UIImageimageNamed:@"logo"];
imgView.layer.shadowColor = [UIColor redColor].CGColor;
imgView.layer.shadowOffset = CGSizeMake(20,20);
imgView.layer.shadowOpacity = 0.1;
[self.viewaddSubview:imgView];
4.不透明，有子视图，并且子视图有内容，会造成离屏渲染
UIImageView*imgView = [[UIImageViewalloc]initWithFrame:CGRectMake(50,50,100,100)];
imgView.image= [UIImageimageNamed:@"logo"];
[self.viewaddSubview:imgView];
imgView.alpha = 0.5;//alpha=1不会触发离屏渲染
imgView.layer.allowsGroupOpacity = YES;
UIView*view = [[UIViewalloc]initWithFrame:CGRectMake(0,0,20,20)];
view.backgroundColor = [UIColor whiteColor];
[imgViewaddSubview:view];
5.UIView设置圆角，有子视图，且子视图有内容，且设置了ClickToBounds=YES则会造成离屏渲染，可以不设置ClickToBounds也可以造成圆角效果，且不会造成离屏渲染
6.UILabel设置圆角，不会造成离屏渲染
7.UIImageView设置圆角，不设置背景色不会造成离屏渲染，设置背景色会
8.UIButton设置圆角，单文本不会造成离屏渲染，设置背景图片会造成离屏渲染
