[iup] Fix mismatch in behaviour with fonttools

This patch fixes an issue with iup not resolving, although I do not
fully understand the problem.

Basically: in comparing our implementation with the one in fonttools,
there is clear divergence in some of the numerical code, where we are
using Option<usize> in places where python is just using an integer.
More importantly, we are using None where Python is using -1, but our
comparison code is never checking if a value == None, whereas python
*does* check if a value == -1, which means that python is setting the
solution in cases where we are not.

This can be fixed by using `checked_sub` instead of `saturating_sub`, at
least for this input.

It feels like this is probably fine, since the structure of this loop is
such that the only negative value that is possible is -1 (at the start)
but it feels a little loose.

write-fonts/src/tables/gvar/iup.rs

@@ -527,7 +527,7 @@ fn iup_contour_optimize(
                 solution.insert(idx % n);
                 i = dp_result.chain[idx];
             }
-            if i == Some(start.saturating_sub(n)) {
+            if i == start.checked_sub(n) {
                 // Python reads [-1] to get 0, usize doesn't like that
                 let cost = dp_result.costs[start]
                     - if n < start {
@@ -1279,4 +1279,65 @@ mod tests {
             ]
         )
     }
+
+    // https://github.com/googlefonts/fontations/issues/662
+    #[test]
+    fn googlesans_m_sc() {
+        let deltas = vec![
+            Vec2 { x: 73.0, y: 0.0 },
+            Vec2 { x: 73.0, y: 580.0 },
+            Vec2 { x: 189.0, y: 580.0 },
+            Vec2 { x: 381.0, y: 116.0 },
+            Vec2 { x: 385.0, y: 116.0 },
+            Vec2 { x: 578.0, y: 580.0 },
+            Vec2 { x: 692.0, y: 580.0 },
+            Vec2 { x: 692.0, y: 0.0 },
+            Vec2 { x: 607.0, y: 0.0 },
+            Vec2 { x: 607.0, y: 342.0 },
+            Vec2 { x: 611.0, y: 448.0 },
+            Vec2 { x: 607.0, y: 448.0 },
+            Vec2 { x: 417.0, y: 0.0 },
+            Vec2 { x: 349.0, y: 0.0 },
+            Vec2 { x: 158.0, y: 448.0 },
+            Vec2 { x: 154.0, y: 448.0 },
+            Vec2 { x: 158.0, y: 342.0 },
+            Vec2 { x: 158.0, y: 0.0 },
+            Vec2 { x: 0.0, y: 0.0 },
+            Vec2 { x: 765.0, y: 0.0 },
+            Vec2 { x: 0.0, y: 0.0 },
+            Vec2 { x: 0.0, y: 0.0 },
+        ];
+
+        let coords = [
+            (73.0, 0.0),
+            (73.0, 580.0),
+            (189.0, 580.0),
+            (381.0, 116.0),
+            (385.0, 116.0),
+            (578.0, 580.0),
+            (692.0, 580.0),
+            (692.0, 0.0),
+            (607.0, 0.0),
+            (607.0, 342.0),
+            (611.0, 448.0),
+            (607.0, 448.0),
+            (417.0, 0.0),
+            (349.0, 0.0),
+            (158.0, 448.0),
+            (154.0, 448.0),
+            (158.0, 342.0),
+            (158.0, 0.0),
+            (0.0, 0.0),
+            (765.0, 0.0),
+            (0.0, 0.0),
+            (0.0, 0.0),
+        ]
+        .into_iter()
+        .map(|(x, y)| Point::new(x, y))
+        .collect::<Vec<_>>();
+
+        let contour_ends = vec![17];
+
+        iup_delta_optimize(deltas, coords, 0.5, &contour_ends).unwrap();
+    }
 }