address review: charge actual clip work instead of projected polygon growth

crates/metafile/src/cliprect.rs

@@ -111,12 +111,10 @@ pub(crate) fn clip_polygon(poly: &[P], clip: &[P]) -> Vec<P> {
 /// can gain a vertex at every edge, so its cost isn't known from its input size alone. `None`
 /// once the budget is spent.
 fn clip_polygon_within(poly: &[P], clip: &[P], budget: &mut usize) -> Option<Vec<P>> {
-    if poly.len().saturating_mul(clip.len()) > *budget {
-        return None;
-    }
     let mut poly = poly.to_vec();
     for i in 0..clip.len() {
-        if poly.len().saturating_mul(clip.len() - i) > *budget {
+        // The work actually done: this edge visits the polygon as it now stands.
+        if poly.len() > *budget {
             return None;
         }
         *budget -= poly.len();

crates/metafile/src/tests.rs

@@ -2075,6 +2075,17 @@ fn clip_polygon_work_bound() {
     assert!(clipped.is_empty(), "excessive polygon clip product rejected");
 }
 
+#[test]
+fn clip_polygon_charges_actual_work_not_projected_growth() {
+    // Two convex 1,000-gons: the product is exactly the budget, and a cut that adds a vertex must
+    // not make a later step look over budget.
+    let gon = |cx: f64| -> Vec<(f64, f64)> {
+        (0..1000).map(|i| (i as f64 / 1000.0 * std::f64::consts::TAU).sin_cos()).map(|(s, c)| (cx + 100.0 * c, 100.0 * s)).collect()
+    };
+    let clipped = crate::cliprect::clip_polygon(&gon(0.0), &gon(30.0));
+    assert!(clipped.len() > 100, "the intersection is kept: {}", clipped.len());
+}
+
 #[test]
 fn clip_work_is_bounded_across_subpaths() {
     use crate::canvas::Raw;