Support layers that generate canvases

Nobody has claimed this yet.

I followed the instructions on http://leafletjs.com/examples/extending/extending-2-layers.html to make my own canvas layer. Sadly this layer does not get drawn when creating the image. It would be nice if this kind of layers could be drawn too.
```javascript
L.GridLayer.CanvasCircles = L.GridLayer.extend({
createTile: function (coords) {
var tile = document.createElement('canvas');

var tileSize = this.getTileSize();
tile.setAttribute('width', tileSize.x);
tile.setAttribute('height', tileSize.y);

var ctx = tile.getContext('2d');

// Draw whatever is needed in the canvas context
// For example, circles which get bigger as we zoom in
ctx.beginPath();
ctx.arc(tileSize.x/2, tileSize.x/2, 4 + coords.z*4, 0, 2*Math.PI, false);
ctx.fill();

return tile;
}
});
```

Here is a fiddle:
https://jsfiddle.net/y0d7jpn4/2/

Start by locating the image-generation path that handles Leaflet layers and compare it with the canvas layer example in the issue and linked fiddle. Trace how returned canvas tiles are discovered and rendered, then verify that a custom L.GridLayer.Canvas layer appears in the generated image without breaking existing layer types.

Written by the indexing model from the issue text.