Handling the GUI

[tgross35]

Brainstorming some ways that we could make the GUI nicely pluggable. I think that we need to be able to decompose to a handful of simple items:

trait Renderable {
    /// Decompose this item into renderable elements
    fn decompose(&self) -> impl Iterator<Item = Render>;
}

trait RenderItem {
    fn hitbox(&self) -> Hitbox;
    fn render(...) -> ...
}

struct Hitbox {
    x: i32,
    y: i32,
    width: i32,
    height: i32
}

Basically, have a quick method (hitbox) that the caller can use to check whether the item is in the viewport, then a slow method to actually get the information needed to render it. The slow method could return something like &dyn Draw or a concrete enum type, I'm not sure what would be best.