Add egui::Scene for panning/zooming a Ui

[grtlr]

This is similar to ScrollArea, but:

• Supports zooming
• Has no scroll bars
• Has no limits on the scrolling

TODO

• Automatic sizing of Scenes outer bounds
• Fix text selection in scenes
• Implement fit_rect
• Document / improve API

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5505-grtlrzoom-and-pan-area
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[emilk]

I wonder if we can come up with a better name for this? 🤔

[grtlr]

Some things that come to my mind (lower entries of the list are augmented by our AI overlords, you might guess which):

• (infinite) Canvas (but might be too close to <canvas />)
• Scene
• View (too general; Viewport is already taken)
• Pad
• Board
• Expanse

[xTibor]

Viewfinder?

This idea comes from photography. Short Wikipedia definition:

... a viewfinder is a device on a camera that a photographer uses to determine exactly where the camera is pointed, and approximately how much of that view will be photographed

I think this kinda describes what this container does.

[YgorSouza]

Viewbox is a term that is sometimes used for this kind of thing, although often it refers to the coordinates of the inner element that are in view, rather than the container widget itself.

[grtlr]

Looks very nice—thank you @emilk for picking this up!

I can't approve my own PR, so here is a ✅.