Misc code cleanup (#3398)

* eframe README: explain how to enable copy/paste

* Implement Debug for a couple of structs

* Code cleanup

* Better docs

* profile ron serialization

* CI: Allow "exclude from changelog" as the only label

crates/epaint/src/shape.rs

@@ -498,6 +498,8 @@ pub struct RectShape {
     ///
     /// To display a texture, set [`Self::fill_texture_id`],
     /// and set this to `Rect::from_min_max(pos2(0.0, 0.0), pos2(1.0, 1.0))`.
+    ///
+    /// Use [`Rect::ZERO`] to turn off texturing.
     pub uv: Rect,
 }