[Multiscene] Adding/Removing Scenes In Background

[TNTErick]

First, thanks for the awesome project.

The current behaviour is there will be a spinning widget whenever a splat-scene is added or removed. And while we can hide the spinning widget, there is nothing we can do when it is still loading.

It would be ideal that maybe we can keep rendering the old splat scenes until the new one has done.

Is there any solution to this right now? Or is it necessary for the viewer to block out the entire render loop?