Make non-visible viewport layer updates configurable

[seankmartin]

I'm not sure what the correct behavior should be. The result of update on a layer in a non-visible viewport may depend on why it's not visible, because it will try to calculate visible chunks, etc. Right now I think viewportIsVisible means the viewport has non-zero size and overlaps the canvas.

So I think we have a couple of options:

1. call update() on layers in non-visible viewports, use CSS (visibility: hidden or whatever, I can't remember) to hide viewports. Viewports/layers can maintain "visible" chunks without actually rendering, so they're ready to render if/when they become visible again.
2. don't call update() on non-visible viewports, they will maintain the renderable objects they had when hidden (not sure if there is a race here)

Originally posted by @aganders3 in #622 (comment)

[seankmartin]

I modified the multi-viewport example to allow toggling between showing the volume and slice viewport and making the shown one fullscreen, or showing both in half and half to get a feel for this. I separated t for each viewport, as otherwise at LOD 2 on volume rendering both fetch very similar data. At higher resolution LODs for volume rendering this would be a bit different

Here is the version that calls update on non-visible viewports:
https://github.com/user-attachments/assets/39b5cdb5-c68a-45f7-a10d-6a4a704d18f9

And the version that does not call update on non-visible viewports:
https://github.com/user-attachments/assets/958eabd2-d10a-4d1e-a2dd-b393c561d3db

Both feel pretty reasonable (I ran the non-update first so cache would be a bit better on the always update version), but it can be nice in the version that does call update how quickly you can see the result. If someone was playing back through time while changing visible viewports this could feel pretty good.

It might be best to consider to allow the update to be configurable on the viewport. Because I'm not sure we can accurately infer why the viewport is non-visible and what the desired behaviour for that viewport should be

[aganders3]

Closed by #653