Tiebreaker for snap-rounding examination order

polygon.cpp

@@ -323,10 +323,14 @@ struct scan_transition {
 	bool operator<(scan_transition const &s) const {
 		if (y < s.y) {
 			return true;
-		} else {
+		} else if (y == s.y) {
-			return false;
+			if (segment < s.segment) {
+				return true;
 			}
 		}
+
+		return false;
+	}
 };
 
 void snap_round(std::vector<segment> &segs, long long extent) {