Add possibility for implementing a renderer?

Hello,

This crate looks super cool so thanks for that.
What do you think about adding the possibility for users of this crate to implement their own `Renderer`?

One use-case would be people that don't use glium/webgpu.rs (I use [gl](https://docs.rs/gl/0.14.0/gl/) for my game). I woud build my own `GLRenderer`, but as it is now, the `Renderer` trait is impossible to implement as it depends on things that are private like `TextureHandle`, `FontHandle`, `FontSource`, etc.

Thanks,
Alex