Fix bug with area function

src/area.rs

@@ -4,12 +4,15 @@ use geo_types::Coord;
 /// We expect the ring to be closed, i.e. the first and last points are the same
 /// (this is not checked because we already know it's true due to the way we
 /// construct the rings in the trace_band_paths function).
+/// Note that in the shoelace formula you need to divide this result by 2 to get the actual area.
+/// Here we skip this division because we only use this area formula to calculate the winding
+/// order of polygons and to compare their relative sizes.
 pub(crate) fn area(ring: &[Coord<f64>]) -> f64 {
     let n = ring.len();
     if n < 3 {
         return 0.;
     }
-    let mut area = ring[n - 2].y * ring[0].x - ring[n - 2].x * ring[0].y;
+    let mut area = ring[n - 1].y * ring[0].x - ring[n - 1].x * ring[0].y;
     for pts in ring.windows(2) {
         area += pts[0].y * pts[1].x - pts[0].x * pts[1].y;
     }