OctreeLoader does not reuse Workers when loading a new pointcloud

[Jordan-Lane]

When loading a new Potree2 model, the OctreeLoader class does not re-use existing Workers. This means that everytime a new Pointcloud is added, both a new WorkerPool and new Workers are created for that specific PointCloud. This is a performance issue, and could cause the browser to crash if too many PointClouds are generated over the lifetime of the app.

You can easily reproduce this issue with the example in this repo. Load the example, and repeatedly 'Load' and 'Unload' the Potree2 model. You will see new Workers are generated every time the PointCloud is loaded. I was able to easily generate 50+ workers by toggling the PointCloud on and off:

[Jordan-Lane]

There are two different solutions I have created and opened as PRs.

The first, and more basic solution is #181. This fixes the issue, however it does not fix other problems that come with having multiple WorkerPools.

The second, and better solution is: #182. This solution creates a single WorkerPool to be used across all PointClouds.

I would much prefer to merge the second solution in, however I also want to fix this issue. Please let me know if there are any changes / improvements you would like to see so that we can get this merged in!