feat(ImageSlice): add forceOpacity and forceTransparent

[PaulHax]

To maintain consistent render effect with stacked image slices no matter the opacity of the slices.

Context

When the prop's transparency crosses from .999 to 1, it gets slotted in the Opaque render pass and can change its render order: https://github.com/Kitware/vtk-js/blob/master/Sources/Rendering/Core/ImageSlice/index.js#L24-L26

When multiple slices are layered changing the tranparency can inavertently change how they are blended. Would be nice to keep a slice in a consistent render pass despite the opacity. Then can avoid this workaround: https://github.com/Kitware/VolView/pull/654/files#diff-3b32a6ae4684ef94faf56b59ea525d8c8adccc792d96cb56613eec2aedf6faf6R20

Results

From an added test: before with 2 actors where one is translucent

  actorBelow.getProperty().setOpacity(0.9);
  actorAbove.getProperty().setOpacity(1);

After with adding actorAbove.setForceTranslucent(true)

Changes

• Documentation and TypeScript definitions were updated to match those changes

PR and Code Checklist

• semantic-release commit messages
• Run npm run reformat to have correctly formatted code

Testing

• This change adds or fixes unit tests

[finetjul]

LGTM

[github-actions]

🎉 This PR is included in version 32.3.0 🎉

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[sedghi]

Is this a workaround for a bug in rendering? I'm trying to understand when to force and when not to force?

[finetjul]

You may want to forceOpaque when you want to render something translucent but writing in the depth buffer.
You may want to forceTransparent when you want to render something opaque without writing in the depth buffer.

[sedghi]

Thanks @finetjul

I guess the default (without setting these force properties) is to render something translucent without a depth buffer and render something opaque with writing to the depth buffer?

What are the main reasons that one might or might not want to write to the depth buffer?

[finetjul]

I guess the default (without setting these force properties) is to render something translucent without a depth buffer and render something opaque with writing to the depth buffer?

Yes, this is the default.

What are the main reasons that one might or might not want to write to the depth buffer?

Want to write in depth buffer: if you don't want other actors to render behind the actor
Don't want to write in depth buffer: if you want other actors to render behind/in front of the actor

I do not have a special use case in mind (besides Paul's), but I remember needing those in VTK C++. That's why Paul just "brought" those "forceXXX" functions from VTK-C++ to VTK.js

[sedghi]

Amazing, thanks