feat: always store volume rendering depth samples in JSON state if volume rendering is enabled

[seankmartin]

Always store the number of depth samples from volume rendering in the JSON state if volume rendering is enabled. Now the JSON state should contain the number of volume rendering samples if it was either changed from the default, or never changed from the default, but volume rendering is active.

This should allow us to make future changes to the volume rendering number of samples more readily without affecting existing states that have volume rendering in use.

[jbms]

I'd be inclined to bump the default for new layers but not modify existing states.

[seankmartin]

Thanks @jbms, I changed the scope of this PR to instead focus on always storing the number of depth samples from volume rendering in the JSON state if volume rendering is enabled. So now the JSON state should contain the number of volume rendering samples if it was either changed from the default, or never changed from the default, but volume rendering is active.

This should allow us to make future changes to the volume rendering number of samples more readily without affecting existing states that have volume rendering in use

[chrisj]

On preserving state behavior, should we consider removing TrackableValue's return of undefined when the value is the same as the default? State size is one concern. If trackablevalue.tojson had a force argument, it would make this PR a little cleaner

return this.volumeRenderingDepthSamplesTarget.toJSON(
  isVolumeRenderingActive,
);

[seankmartin]

On preserving state behavior, should we consider removing TrackableValue's return of undefined when the value is the same as the default? State size is one concern. If trackablevalue.tojson had a force argument, it would make this PR a little cleaner

return this.volumeRenderingDepthSamplesTarget.toJSON(
  isVolumeRenderingActive,
);

That's a great idea for the force parameter thank you Chris. Maybe it could be configurable at compile time the default force value and then up to the app whether to store values always or only if non default?

Though a lot of classes just implement being a Trackable so this would need to be enforced quite broadly across the codebase

[chrisj]

Good point, would require a lot of changes, for example

neuroglancer/src/display_context.ts

Line 400 in 64c47bd

return undefined;

Would need to add a !force check where it returns undefined