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/text/text_layout.rs

@@ -3,7 +3,7 @@ use std::sync::Arc;
 
 use emath::*;
 
-use crate::{text::font::Font, Color32, Mesh, Stroke, Vertex};
+use crate::{stroke::PathStroke, text::font::Font, Color32, Mesh, Stroke, Vertex};
 
 use super::{FontsImpl, Galley, Glyph, LayoutJob, LayoutSection, Row, RowVisuals};
 
@@ -853,7 +853,7 @@ fn add_hline(point_scale: PointScale, [start, stop]: [Pos2; 2], stroke: Stroke,
         let mut path = crate::tessellator::Path::default(); // TODO(emilk): reuse this to avoid re-allocations.
         path.add_line_segment([start, stop]);
         let feathering = 1.0 / point_scale.pixels_per_point();
-        path.stroke_open(feathering, stroke, mesh);
+        path.stroke_open(feathering, &PathStroke::from(stroke), mesh);
     } else {
         // Thin lines often lost, so this is a bad idea