geometry: rewrite Coordinates trait into a concrete struct, pass coords by reference to shaders, and add a Rectangle type

src/render.rs

@@ -14,7 +14,7 @@ use std::marker::PhantomData;
 use std::fmt::Debug;
 
 pub trait Shader: Send + Debug {
-    fn draw(&self, surface_coords: VirtualCoordinates) -> RGB8;
+    fn draw(&self, surface_coords: &VirtualCoordinates) -> RGB8;
 }
 
 pub trait Surface: Default + Clone + Debug {