根据 WebGL最新规范，你现在需要调用 getContext 设置 preserveDrawingBuffer flag，如：

According to WebGL latest specs, you now need to call getContext setting the preserveDrawingBuffer flag, like:

var ctx = canvas.getContext("experimental-webgl", {preserveDrawingBuffer: true});

这可以防止绘图缓冲区（颜色，深度，模板）在绘制到屏幕后被清除。请记住，这可能会导致性能下降。

this prevents the drawing buffer (color, depth, stencil) from being cleared after they are draw to screen. Keep in mind that settings this may cause a performance penalty.