Node corruption while zooming out

[e-kud]

I found the following bug that can be observed at the http://evmar.github.io/webtreemap/ page. It irrecoverably deletes nodes while zooming out.

To reproduce we need to make three clicks on the page.

1. Click on the uglify_js/node_modules square. Now we can see CHANGELOG.md, README.md and other files.
2. Click on the most outer block node_modules (50.0m)
3. Click again on the uglify_js/node_modules and there are no files that we have seen at the point 1.

I think the problem might be in this loop: https://github.com/evmar/webtreemap/blob/e238c45fb41867c53775a361ee0e007c82d070ac/src/treemap.ts#L265

[evmar]

Thanks for the great bug report! I confirmed it and I will need to figure out a fix now.