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/epaint/src/shape_transform.rs

@@ -1,7 +1,12 @@
+use std::sync::Arc;
+
 use crate::*;
 
 /// Remember to handle [`Color32::PLACEHOLDER`] specially!
-pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
+pub fn adjust_colors(
+    shape: &mut Shape,
+    adjust_color: impl Fn(&mut Color32) + Send + Sync + Copy + 'static,
+) {
     #![allow(clippy::match_same_arms)]
     match shape {
         Shape::Noop => {}
@@ -10,8 +15,48 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
                 adjust_colors(shape, adjust_color);
             }
         }
-        Shape::LineSegment { stroke, points: _ } => {
-            adjust_color(&mut stroke.color);
+        Shape::LineSegment { stroke, points: _ } => match &stroke.color {
+            color::ColorMode::Solid(mut col) => adjust_color(&mut col),
+            color::ColorMode::UV(callback) => {
+                let callback = callback.clone();
+                stroke.color = color::ColorMode::UV(Arc::new(Box::new(move |rect, pos| {
+                    let mut col = callback(rect, pos);
+                    adjust_color(&mut col);
+                    col
+                })));
+            }
+        },
+
+        Shape::Path(PathShape {
+            points: _,
+            closed: _,
+            fill,
+            stroke,
+        })
+        | Shape::QuadraticBezier(QuadraticBezierShape {
+            points: _,
+            closed: _,
+            fill,
+            stroke,
+        })
+        | Shape::CubicBezier(CubicBezierShape {
+            points: _,
+            closed: _,
+            fill,
+            stroke,
+        }) => {
+            adjust_color(fill);
+            match &stroke.color {
+                color::ColorMode::Solid(mut col) => adjust_color(&mut col),
+                color::ColorMode::UV(callback) => {
+                    let callback = callback.clone();
+                    stroke.color = color::ColorMode::UV(Arc::new(Box::new(move |rect, pos| {
+                        let mut col = callback(rect, pos);
+                        adjust_color(&mut col);
+                        col
+                    })));
+                }
+            }
         }
 
         Shape::Circle(CircleShape {
@@ -26,12 +71,6 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             fill,
             stroke,
         })
-        | Shape::Path(PathShape {
-            points: _,
-            closed: _,
-            fill,
-            stroke,
-        })
         | Shape::Rect(RectShape {
             rect: _,
             rounding: _,
@@ -40,18 +79,6 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             blur_width: _,
             fill_texture_id: _,
             uv: _,
-        })
-        | Shape::QuadraticBezier(QuadraticBezierShape {
-            points: _,
-            closed: _,
-            fill,
-            stroke,
-        })
-        | Shape::CubicBezier(CubicBezierShape {
-            points: _,
-            closed: _,
-            fill,
-            stroke,
         }) => {
             adjust_color(fill);
             adjust_color(&mut stroke.color);