vello_toy: Color lines according to their winding sign (#846)

#845 removed line-tile
intersections from the debug rendering, as these are no longer present
in the internal representation.

However, that also removed a way to see whether lines are directed
upwards or downwards, which is important to understand their effect on
winding.

This proposes coloring the lines according to their orientation: lines
oriented upwards add to winding and are colored green. Lines oriented
downwards subtract from winding and are colored red. Horizontal lines
don't affect winding, and will with future changes likely not generate
any tiles at all. These are colored grey.

An example of what that looks like:

```shell
cargo run --bin debug -- --path "M 5 5 L 40 23 L 20 43.5 L 1 43.5 Z"  --stages ls,ta
```


![debug](https://github.com/user-attachments/assets/ee4600a8-eb03-4cef-9796-a86cafec9526)

Author: tomcur

sparse_strips/vello_toy/src/debug.rs

@@ -12,7 +12,7 @@ use clap::Parser;
 use std::collections::HashSet;
 use std::path;
 use svg::node::element::path::Data;
-use svg::node::element::{Path, Rectangle};
+use svg::node::element::{Line as SvgLine, Path, Rectangle};
 use svg::{Document, Node};
 use vello_common::coarse::{Cmd, Wide, WideTile};
 use vello_common::color::palette::css::BLACK;
@@ -140,31 +140,33 @@ fn draw_grid(document: &mut Document, width: u16, height: u16) {
 }
 
 fn draw_line_segments(document: &mut Document, line_buf: &[Line]) {
-    let mut data = Data::new();
-
-    let mut last = None;
+    let svg_line = SvgLine::new()
+        .set("stroke-width", 0.1)
+        .set("fill", "none")
+        .set("fill-opacity", 0.1);
 
     for line in line_buf {
-        let first = (line.p0.x, line.p0.y);
-        let second = (line.p1.x, line.p1.y);
-
-        if Some(first) != last {
-            data = data.move_to(first);
-        }
-
-        data = data.line_to(second);
+        let dy = line.p1.y - line.p0.y;
+        let color = if dy < 0. {
+            // Lines oriented upwards add to winding.
+            "green"
+        } else if dy > 0. {
+            // Lines oriented upwards subtract from winding.
+            "red"
+        } else {
+            // Horizontal lines don't impact winding.
+            "grey"
+        };
 
-        last = Some(second);
+        let svg_line = svg_line
+            .clone()
+            .set("x1", line.p0.x)
+            .set("y1", line.p0.y)
+            .set("x2", line.p1.x)
+            .set("y2", line.p1.y)
+            .set("stroke", color);
+        document.append(svg_line);
     }
-
-    let border = Path::new()
-        .set("stroke-width", 0.1)
-        .set("stroke", "green")
-        .set("fill", "none")
-        .set("fill-opacity", 0.1)
-        .set("d", data);
-
-    document.append(border);
 }
 
 fn draw_tile_areas(document: &mut Document, tiles: &Tiles) {