shadow improve + performance step 2.1 (focus on Path)

Runtime/flow/physical_shape_layer.cs

         public static void drawShadow(Canvas canvas, Path path, Color color, float elevation, bool transparentOccluder,
             float dpr) {
             float kAmbientAlpha = 0.039f;
-            float kSpotAlpha = 0.25f;
+            float kSpotAlpha = ShadowUtils.kUseFastShadow ? 0.1f : 0.25f;
             float kLightHeight = 600f;
             float kLightRadius = 800f;
 

Runtime/ui/painting/path.cs

                     }
                 }
             }
-
             // Split arc into max 90 degree segments.
             int ndivs = Mathf.Max(1, Mathf.Min((int) (Mathf.Abs(da) / (Mathf.PI * 0.5f) + 0.5f), 5));
             float hda = (da / ndivs) / 2.0f;
                 this.addPoint(0, 0, PointFlags.corner);
             }
 
-            var path = this._paths.Last();
+            var path = this._paths[this._paths.Count - 1];
-                var pt = this._points.Last();
+                var pt = this._points[this._points.Count - 1];
                 if (PathUtils.ptEquals(pt.x, pt.y, point.x, point.y, this._distTol)) {
                     pt.flags |= point.flags;
                     return;
                 y1 = 0;
             }
             else {
-                var pt = this._points.Last();
+                var pt = this._points[this._points.Count - 1];
                 x1 = pt.x;
                 y1 = pt.y;
             }
                 return;
             }
 
-            var path = this._paths.Last();
+            var path = this._paths[this._paths.Count - 1];
             path.winding = winding;
         }