lib8: drop custom RGB8 struct for rgb crate

src/render.rs

@@ -1,11 +1,11 @@
 use std::rc::Rc;
 use std::cell::RefCell;
 use std::io;
+use rgb::RGB8;
 
 #[cfg(feature="threads")]
 use std::sync::{Arc, Mutex};
 
-use crate::lib8::RGB8;
 use crate::geometry::*;
 
 pub trait Shader: Send {