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/stroke.rs

@@ -1,5 +1,7 @@
 #![allow(clippy::derived_hash_with_manual_eq)] // We need to impl Hash for f32, but we don't implement Eq, which is fine
 
+use std::{fmt::Debug, sync::Arc};
+
 use super::*;
 
 /// Describes the width and color of a line.
@@ -52,3 +54,68 @@ impl std::hash::Hash for Stroke {
         color.hash(state);
     }
 }
+
+/// Describes the width and color of paths. The color can either be solid or provided by a callback. For more information, see [`ColorMode`]
+///
+/// The default stroke is the same as [`Stroke::NONE`].
+#[derive(Clone, Debug, Default, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+pub struct PathStroke {
+    pub width: f32,
+    pub color: ColorMode,
+}
+
+impl PathStroke {
+    /// Same as [`PathStroke::default`].
+    pub const NONE: Self = Self {
+        width: 0.0,
+        color: ColorMode::TRANSPARENT,
+    };
+
+    #[inline]
+    pub fn new(width: impl Into<f32>, color: impl Into<Color32>) -> Self {
+        Self {
+            width: width.into(),
+            color: ColorMode::Solid(color.into()),
+        }
+    }
+
+    /// Create a new `PathStroke` with a UV function
+    ///
+    /// The bounding box passed to the callback will have a margin of [`TessellationOptions::feathering_size_in_pixels`](`crate::tessellator::TessellationOptions::feathering_size_in_pixels`)
+    #[inline]
+    pub fn new_uv(
+        width: impl Into<f32>,
+        callback: impl Fn(Rect, Pos2) -> Color32 + Send + Sync + 'static,
+    ) -> Self {
+        Self {
+            width: width.into(),
+            color: ColorMode::UV(Arc::new(callback)),
+        }
+    }
+
+    /// True if width is zero or color is solid and transparent
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.width <= 0.0 || self.color == ColorMode::TRANSPARENT
+    }
+}
+
+impl<Color> From<(f32, Color)> for PathStroke
+where
+    Color: Into<Color32>,
+{
+    #[inline(always)]
+    fn from((width, color): (f32, Color)) -> Self {
+        Self::new(width, color)
+    }
+}
+
+impl From<Stroke> for PathStroke {
+    fn from(value: Stroke) -> Self {
+        Self {
+            width: value.width,
+            color: ColorMode::Solid(value.color),
+        }
+    }
+}