小游戏渲染模式为Webgl时，canvas.toTempFilePath截图保存图片后，图片有严重的边缘残影。渲染模式为canvas时，没有这个问题。

在小游戏里是正常的，显示没有任何问题。

代码：

            var canvas = cc.game.canvas;

            var height  = canvas.height;

            var width  = canvas.width;

            canvas.toTempFilePath({

                x: 0,

                y: 0,

                width: width,

                height: height,

                destWidth: width,

                destHeight: height,

                success (res) {

                    console.log(res);

                    wx.saveImageToPhotosAlbum({

                        filePath: res.tempFilePath,

                        success:function(r){

                            console.log(r);

                            cc.instantiate(tha