canvas.draw_image is slow

[martinRenou]

The slow part in ipycanvas is here when we use an Image widget: https://github.com/martinRenou/ipycanvas/blob/master/src/widget.ts#L786-L789
the slowness comes from this function https://github.com/martinRenou/ipycanvas/blob/658929381918dd89dc7f38a735fa7f9148a85f2b/src/widget.ts#L43-L62 which transforms the Image widget into an actual JavaScript HTMLImageElement object

We could probably do it only once for each image widget (keeping the resulting HTMLImageElement in memory in a {"widget-model-id": HTMLImageElement} object, we can also automatically update this mapping if we see changes in the Image model).

That way animating an image would be more performant.