Render: compound outlines (double, thick-thin, thin-thick, triple) (#287)

a:ln/@cmpd was read but every outline was drawn as one band. Each band is
now stroked along the path moved to its place across the width, in the
shares PowerPoint uses (measured on its export of 24 pt lines: double
1/3-1/3-1/3, thin-thick 1/5-1/5-3/5, triple 1/6-1/6-1/3-1/6-1/6), the
first band on the left of travel (the outside of a shape). Line corners
meet at their miter points; curves are offset with kurbo.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/render/src/tests.rs

@@ -776,3 +776,47 @@ fn pattern_fills_draw_powerpoints_tiles() {
         assert_eq!(dark(x, y), dark(x + 1, y + 1), "pct50 repeats diagonally at {x},{y}");
     }
 }
+
+#[test]
+fn compound_lines_draw_their_bands() {
+    // A 24 pt outline on a rectangle, as PowerPoint's export draws each kind (top edge, outside
+    // first): double 8/8/8, thin-thick 4.8/4.8/14.4, triple 4/4/8/4/4.
+    use deckcraft_model::style::{Compound, Fill, Line};
+    let runs = |c: Compound| {
+        let sh = Shape {
+            id: ShapeId(760),
+            xfrm: Some(Xfrm::new(100.0, 100.0, 300.0, 200.0)),
+            fill: Some(Fill::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(255, 255, 255)))),
+            line: Some(Line {
+                fill: Some(Fill::solid(ColorRef::rgb(deckcraft_color::Rgba::rgb(0, 0, 0)))),
+                width: Some(24.0),
+                compound: Some(c),
+                ..Default::default()
+            }),
+            ..Default::default()
+        };
+        let p = deck_with(vec![sh]);
+        let img = render_slide(&p, 0, &RenderOpts { scale: 2.0, ..Default::default() });
+        // Down the middle of the top edge (y 100 pt), from 80 pt to 120 pt: lengths of dark and
+        // light runs in points.
+        let mut out: Vec<(bool, f64)> = vec![];
+        for y in 160..240 {
+            let dark = img.pixel(500, y)[0] < 128;
+            match out.last_mut() {
+                Some((d, n)) if *d == dark => *n += 0.5,
+                _ => out.push((dark, 0.5)),
+            }
+        }
+        out.into_iter().filter(|(d, _)| *d).map(|(_, n)| n).collect::<Vec<f64>>()
+    };
+    let close = |got: Vec<f64>, want: &[f64]| got.len() == want.len() && got.iter().zip(want).all(|(g, w)| (g - w).abs() <= 1.0);
+    assert!(close(runs(Compound::Single), &[24.0]));
+    let d = runs(Compound::Double);
+    assert!(close(d.clone(), &[8.0, 8.0]), "{d:?}");
+    let t = runs(Compound::ThinThick);
+    assert!(close(t.clone(), &[4.8, 14.4]), "{t:?}");
+    let k = runs(Compound::ThickThin);
+    assert!(close(k.clone(), &[14.4, 4.8]), "{k:?}");
+    let tri = runs(Compound::Triple);
+    assert!(close(tri.clone(), &[4.0, 8.0, 4.0]), "{tri:?}");
+}