stretchableImageWithLeftCapWidth气泡拉伸
- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapHeight;
从点(topCapHeight,topCapHeight)往右下角開始拉伸
- (UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCapHeight:(NSInteger)topCapHeight;
从点(topCapHeight,topCapHeight)往右下角開始拉伸