icon: refactor Icon to be dyn

Same as for `CustomCursor`. However, the API uses `dyn` stuff only
because of `Windows` backend at the time of writing, generally, platforms
should just have a separate method that deals with all of that, and e.g.
top-level winit only guarantees `Rgba`.

src/window.rs

@@ -46,7 +46,7 @@ impl fmt::Debug for WindowId {
 }
 
 /// Attributes used when creating a window.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, Clone)]
 pub struct WindowAttributes {
     pub surface_size: Option<Size>,
     pub min_surface_size: Option<Size>,