Tracking issue: Misc improvements to multi-viewports

[emilk]

Multi-viewports were implemented in https://github.com/emilk/egui/pull/3172, but there are several features missing that would make it a lot nicer, including (in no particular order):

• [ ] Fix buggy text: https://github.com/emilk/egui/issues/3664
• [ ] Fix texture management
• [ ] Make it easy to wrap child viewports in the same chrome as crate::Window
• [ ] Automatically show embedded viewports using crate::Window
• [x] Use the new ViewportBuilder in eframe::NativeOptions
• [ ] Automatically position new viewport windows (they currently cover each other)
• [x] Add a Context method for listing all existing viewports
• [ ] Simplify the code related to Memory
• [ ] Store viewport info (position, size, …) in egui memory
• [x] Add more info to ViewportInfo
• [x] Supply a ViewportInfo for every viewport in RawInput
• [ ] Support drag-dropping items between viewports
• [ ] Proper docking: https://github.com/rerun-io/egui_tiles/issues/30
• [ ] Support in eframe web
• …

[juancampa]

Not sure if out of scope for this issue but one that I'm particularly interested in:

• Implement viewports for the web backed (should the app provide a create_canvas_for_viewport callback?)

My use case for this, btw, is improving performance of the Membrane extension by splitting the app into canvases that can be re-rendered independently.

[emilk]

That defiantly belongs here! We can break out smaller issues from this one as people start working on them