Use egui-glow instead of custom implementation of egui

[asny]

• Deprecate egui implementation
• Use egui-glow in examples

[asny]

It's easy to use egui-glow with a winit window, but not on web. So there are several options as I see it:

• Use egui-glow Painter and implement event mapping etc. in every example that requires gui
• Use eframe instead of a three-d window for those examples that requires gui
• Keep an implementation of egui (use egui-glow for native and a custom for web)
• Make a contribution to egui to make egui-glow support web
• Only have a gui on native

[emilk]

egui_glow::Painter is already being used for web by egui_web, so it should not be too much work to use egui_glow::Painter in three-d.

[asny]

I decided to keep an egui implementation to avoid duplicating the necessary code everywhere. However, that implementation is using egui-glow so it is a really thin wrapper, most of the code is for mapping from three-d input (for example events) to egui input.