add canvas.toDataURL method

    // support PNG only.
    // JPEG doesn't support the alpha channel , it's not commonly used in Game Developing.
    // PNG is enough in most cases.

example code 
```
var bufferCanvas=document.createElement("canvas");
var bufferContext=bufferCanvas.getContext("2d");
bufferContext.fillStyle="red";

var img=new Image();
img.src="./res/test.png";
img.onload=function(){
    bufferCanvas.width=img.width;
    bufferCanvas.height=img.height;
    bufferContext.drawImage(img,0,0);

    var url=bufferCanvas.toDataURL();

    console.log(url)

}
```