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/color.rs

@@ -0,0 +1,48 @@
+use std::{fmt::Debug, sync::Arc};
+
+use ecolor::Color32;
+use emath::{Pos2, Rect};
+
+/// How paths will be colored.
+#[derive(Clone)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+pub enum ColorMode {
+    /// The entire path is one solid color, this is the default.
+    Solid(Color32),
+
+    /// Provide a callback which takes in the path's bounding box and a position and converts it to a color.
+    /// When used with a path, the bounding box will have a margin of [`TessellationOptions::feathering_size_in_pixels`](`crate::tessellator::TessellationOptions::feathering_size_in_pixels`)
+    ///
+    /// **This cannot be serialized**
+    #[cfg_attr(feature = "serde", serde(skip))]
+    UV(Arc<dyn Fn(Rect, Pos2) -> Color32 + Send + Sync>),
+}
+
+impl Default for ColorMode {
+    fn default() -> Self {
+        Self::Solid(Color32::TRANSPARENT)
+    }
+}
+
+impl Debug for ColorMode {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Solid(arg0) => f.debug_tuple("Solid").field(arg0).finish(),
+            Self::UV(_arg0) => f.debug_tuple("UV").field(&"<closure>").finish(),
+        }
+    }
+}
+
+impl PartialEq for ColorMode {
+    fn eq(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Self::Solid(l0), Self::Solid(r0)) => l0 == r0,
+            (Self::UV(_l0), Self::UV(_r0)) => false,
+            _ => false,
+        }
+    }
+}
+
+impl ColorMode {
+    pub const TRANSPARENT: Self = Self::Solid(Color32::TRANSPARENT);
+}