Fix negative sweep angle normalization

Author: tomcur

src/arc.rs

@@ -220,7 +220,7 @@ impl ParamCurveArclen for Arc {
         // Normalize sweep angle to be non-negative
         let mut sweep_angle = self.sweep_angle;
         if sweep_angle < 0. {
-            start_angle -= sweep_angle;
+            start_angle = PI - start_angle;
             sweep_angle = -sweep_angle;
         }