Support replacing node when filter

[compilelife]

This pull request introduce supporting for replacing node when doing filter.

The original demand is to use first frame of video instead of blank(transparent) when capturing a node.

See what it can do :

Assuming that we have a node:

<div>
   <p>Hello Video</p>
   <video src='http://mock.video.addr.com/1.mp4'></video>
</div>

A regular call to domtoimage.toPng(theDiv) would result to a image only contain the 'Hello world' text, leave blank for video area.

Now you can do:

const convertVideoToCanvas = (node)=>{
  if (node instanceof HTMLVideoElement) {
    const c = document.createElement('canvas');
    c.width = node.clientWidth;
    c.height = node.clientHeight;
    c.setAttribute('style', c.getAttribute('style') || '')
    const ctx = c.getContext('2d');
    if (ctx) {
      if (node.readyState >= 3) {
        ctx.drawImage(node, 0, 0);
      }
    }
    return c;
  }
  return node
}
const options = {
  filter: convertVideoToCanvas
}
domtoimage.toPng(theDiv, option)

This will create a image contains both text and video.

And you can do other similar node replacing trick.

On the other hand, this patch keep the api compatibility of filter unchanged.

So you can still do boolean filter like you always do:

const option = {
  filter: (node)=>{return node.nodeName !== 'video'}
}