Fix potential self-intersection polygon in Scene._filter_structures_plane

Author: weiliangjin2021

tidy3d/components/scene.py

@@ -637,7 +637,7 @@ def _filter_structures_plane(
                 if _shape.is_empty or not shape.intersects(_shape):
                     continue
 
-                diff_shape = _shape - shape
+                diff_shape = (_shape - shape).buffer(0)
 
                 # different prop, remove intersection from background shape
                 if prop != _prop and len(diff_shape.bounds) > 0: