Attempt to integrate iced_native UI for use with Kiss3d

[alvinhochun]

This implements a hacky and incomplete iced_native UI support built on top the generic version of PR #233.

A short write-up on this PR:

• A lot of code was adapted from the iced_glow implementation, some unmodified
• The glow::Context is obtained in a hackish way from Kiss3d (this would be a non-issue if Kiss3d switches to glow in the future as in the web-sys branch)
• Kiss3d assumes a single VAO but the iced_glow rect.rs implementation uses its own VAO, so extra code had to be added to have Kiss3d rebind its VAO
• The code flow and demo example mostly follows iced's integration example
• iced_native does not handle input events in a way that can support integration well (How to integrate input events with existing engine, and use iced as ui library? iced-rs/iced#408), which means there is currently no way to tell whether an iced widget are capturing the mouse or keyboard and there is no way to determine whether the input events are unhandled by iced.

To try the demo, edit gui/iced/Cargo.toml to point to a local copy of iced (use my fork event-capture branch at https://github.com/alvinhochun/iced/tree/event-capture) and run cargo run -p kiss3d_iced --example demo