render: force surfaces to implement default+clone, for now

src/render.rs

@@ -10,7 +10,7 @@ pub trait Shader: Send {
     fn draw(&self, surface_coords: VirtualCoordinates) -> RGB8;
 }
 
-pub trait Surface {
+pub trait Surface: Default + Clone {
     fn with_shader<F: FnMut(&dyn Shader)>(&self, f: F);
     fn set_shader(&mut self, shader: Box<dyn Shader>);
     fn clear_shader(&mut self);