设置控件圆角，只设置上面两个角的圆角，下面两个角依然是直角，如图效果：
代码（通过贝塞尔曲线重绘layer层）：
UIImageView *picImageView = [[UIImageView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
picImageView.backgroundColor = [UIColor greenColor];
[self.view addSubview:picImageView];
// UIRectCornerBottomRight通过这个设置
UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:picImageView.bounds
byRoundingCorners:UIRectCornerTopLeft|UIRectCornerTopRight
cornerRadii:CGSizeMake(10, 10)];
CAShapeLayer *maskLayer = [CAShapeLayer layer];
maskLayer.frame = picImageView.bounds;
maskLayer.path = maskPath.CGPath;
picImageView.layer.mask = maskLayer;
