map地图组件上绘制2个marker点，在2个marker点经纬度都不改变情况下：每次移动map画布时 调用MapContext.toScreenLocation() 返回2个点的坐标x,y。多次调用有时返回的2个marker点的相对坐标x,y 会有1像素的误差。

            for (let i in list) {
                wx.createMapContext('myMap', that).toScreenLocation({
                    longitude: list[i].longitude,
                    latitude: list[i].latitude,
                    success(res) {
                        list[i].x = res.x
                        list[i].y = res.y
                        console.log(i, '<=========toScreenLocation的下标')
                        console.log(res)
                    }
                })
            }