使 Canvas 画布支持高分屏（High DPI）
上一回我们说到用 Canvas 去渲染图像，但是在高分屏中，我们会发现，同样的大小，在高分屏上显示的并不清晰，这个 demo 表示的非常清楚：https://jsfiddle.net/csvwolf/3gvs44vw/
为什么会这样
实际上我们在屏幕中看到的都是逻辑像素，但对于设备中元素的实际像素，是逻辑像素 * 设备像素比
，也可以叫做物理像素。
实际上在 Canvas 绘制时，也需要绘制实际像素，再进行画布的缩放。
解决方案
在 demo 中已经有了方案，先绘制逻辑像素 * 设备像素比大小的画布，用 CSS 把样式大小调整，这样实际上是把 canvas 像素点压缩到指定尺寸，但是这样字会变小，我们还需要利用缩放放大设备像素比倍的尺寸，让尺寸恢复正常。
这样一个支持高分屏的渲染就做完了。
接下来我们考虑，如果我们支持完高分屏，图片像素的坐标要怎么选定：答案是要扩大像素比倍进行选择，因为画布的像素点总数没有变化。
很惭愧，一点微小的实践经验，时间有限，就说这么多。
参考
植入部分
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