給圖片添加水印

#pragma mark -添加水印
/**
	加半透明水印
	@param useImage 需要加水印的圖片
	@param maskImage 水印
	@returns 加好水印的圖片
 */
+ (UIImage *)addImage:(UIImage *)useImage addMsakImage:(UIImage *)maskImage msakRect:(CGRect)rect
{
    UIGraphicsBeginImageContext(useImage.size);
    [useImage drawInRect:CGRectMake(0, 0, useImage.size.width, useImage.size.height)];
    
    //四個參數爲水印圖片的位置
    [maskImage drawInRect:rect];
    UIImage *resultingImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return resultingImage;
}