Handle overlapping gradient stops for linear gradients

From `addColorStop(offset, color)`s spec:
If multiple stops are added at the same offset on a gradient, they must
be placed in the order added, with the first one closest to the start of
the gradient, and each subsequent one infinitesimally further along
towards the end point (in effect causing all but the first and last stop
added at each point to be ignored)

https://www.w3.org/html/test/results/2dcontext/annotated-spec/canvas.html#testrefs.2d.gradient.interpolate.overlapu

components/canvas/raqote_backend.rs

@@ -243,15 +243,19 @@ impl GenericDrawTarget for raqote::DrawTarget {
         dt.get_data_mut().copy_from_slice(s);
         raqote::DrawTarget::copy_surface(self, &dt, source.to_box2d(), destination);
     }
+    // TODO(pylbrecht)
+    // unused code?
     fn create_gradient_stops(
         &self,
         gradient_stops: Vec<GradientStop>,
         _extend_mode: ExtendMode,
     ) -> GradientStops {
-        let stops = gradient_stops
+        let mut stops = gradient_stops
             .into_iter()
             .map(|item| item.as_raqote().clone())
-            .collect();
+            .collect::<Vec<raqote::GradientStop>>();
+        // https://www.w3.org/html/test/results/2dcontext/annotated-spec/canvas.html#testrefs.2d.gradient.interpolate.overlap
+        stops.sort_by(|a, b| a.position.partial_cmp(&b.position).unwrap());
         GradientStops::Raqote(stops)
     }
     fn create_path_builder(&self) -> Box<dyn GenericPathBuilder> {
@@ -730,7 +734,10 @@ impl<'a> ToRaqoteSource<'a> for FillOrStrokeStyle {
                 rgba.blue,
             ))),
             LinearGradient(style) => {
-                let stops = style.stops.into_iter().map(|s| s.to_raqote()).collect();
+                let mut stops: Vec<raqote::GradientStop> =
+                    style.stops.into_iter().map(|s| s.to_raqote()).collect();
+                // https://www.w3.org/html/test/results/2dcontext/annotated-spec/canvas.html#testrefs.2d.gradient.interpolate.overlap
+                stops.sort_by(|a, b| a.position.partial_cmp(&b.position).unwrap());
                 let mut gradient = raqote::Gradient { stops };
                 let start = Point2D::new(style.x0 as f32, style.y0 as f32);
                 let end = Point2D::new(style.x1 as f32, style.y1 as f32);