Added ability to define colors at UV coordinates along a path (#4353)

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I had to make a couple types not Copy because closures, but it should'nt
be a massive deal.

I tried my best to make the API change as non breaking as possible.
Anywhere a PathStroke is used, you can just use a normal Stroke instead.
As mentioned above, the bezier paths couldn't be copy anymore, but IMO
that's a minor caveat.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_demo_lib/src/demo/dancing_strings.rs

@@ -1,9 +1,11 @@
-use egui::{containers::*, *};
+use egui::{containers::*, epaint::PathStroke, *};
 
 #[derive(Default)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "serde", serde(default))]
-pub struct DancingStrings {}
+pub struct DancingStrings {
+    colors: bool,
+}
 
 impl super::Demo for DancingStrings {
     fn name(&self) -> &'static str {
@@ -28,6 +30,9 @@ impl super::View for DancingStrings {
             Color32::from_black_alpha(240)
         };
 
+        ui.checkbox(&mut self.colors, "Colored")
+            .on_hover_text("Demonstrates how a path can have varying color across its length.");
+
         Frame::canvas(ui.style()).show(ui, |ui| {
             ui.ctx().request_repaint();
             let time = ui.input(|i| i.time);
@@ -55,7 +60,24 @@ impl super::View for DancingStrings {
                     .collect();
 
                 let thickness = 10.0 / mode as f32;
-                shapes.push(epaint::Shape::line(points, Stroke::new(thickness, color)));
+                shapes.push(epaint::Shape::line(
+                    points,
+                    if self.colors {
+                        PathStroke::new_uv(thickness, move |rect, p| {
+                            let t = remap(p.x, rect.x_range(), -1.0..=1.0).abs();
+                            let center_color = hex_color!("#5BCEFA");
+                            let outer_color = hex_color!("#F5A9B8");
+
+                            Color32::from_rgb(
+                                lerp(center_color.r() as f32..=outer_color.r() as f32, t) as u8,
+                                lerp(center_color.g() as f32..=outer_color.g() as f32, t) as u8,
+                                lerp(center_color.b() as f32..=outer_color.b() as f32, t) as u8,
+                            )
+                        })
+                    } else {
+                        PathStroke::new(thickness, color)
+                    },
+                ));
             }
 
             ui.painter().extend(shapes);