Children should listen to size changes on parents and re-render as needed.

[trusktr]

Currently, resizing the scene by external means (f.e. resizing the container of a <motor-scene> element) does not cause necessary re-rendering to happen. This causes the problem that if something is proportionally sized (which can ultimately depend on the size of the root <motor-scene> element), then mountPoint calculations will fail because the thing who's mountPoint is being set will not have an updated size, so the mountPoint offset will be incorrect.

There is currently no way to do this reliably except with polling (though that will change soon with the ResizeObserver coming to the web soon). So, we'll poll for now.

We need a good way to do this without simply traversing the entire tree and re-rendering everything. We need a way to listen to size changes (and hence why I'm working on issue #54), in order to re-render only specific nodes that depend on the size updates.

This depends on #54 being completed first. This will also solve #19.

[trusktr]

I increased the scope of this issue so that size change events are fired for any node whose size changes, not just the scene. It's really nice, and it's synchronized with the browser's animation loop, so resizing is sharp and smooth (rather than laggy like when re-rendering on window resize events).