Expose image opacity component

[Wumpf]

What

Image opacity (for layering images) is now an exposed component that can be set from blueprint & data store.

Heuristic for opacity is a lot simplified:

• stacked images will no longer be made transparent in general
• only segmentation image will use 0.5 opacity as a fallback if there is any other image in the scene
  Demonstrated here:

Recording.2024-06-25.122634.mp4

---

• prerequisite for Make image transparency explicit & split image visualizers #6548
• almost solves Remove image layering magic & make image layering explicit #2192

Checklist

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[github-actions]

Deployed docs

Commit	Link
0cccb22	https://landing-juenyzc75-rerun.vercel.app/docs