Bonsai: stop merged cut linework leaking classes onto disjoint pieces

In merge_linework_and_add_metadata every closed subpath of an element
group was queued with a reference to the same class set, so when one
subpath was unioned with a neighbouring element the in-place
polygon_classes.update() also rewrote the classes of that element's
other, geometrically unrelated subpaths. A wall split by an opening
could therefore emit a piece carrying a neighbour's GlobalId, status
and metadata classes, styling it from the wrong CSS rules.

Queue a copy of the class set per polygon so each merged polygon owns
its own classes.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/drawing/operator.py

@@ -1708,7 +1708,7 @@ class CreateDrawing(bpy.types.Operator):
                             coords.append(coords[0])
                         if len(coords) > 2 and coords[0] == coords[-1]:
                             is_closed_polygon = True
-                            queue.append((shapely.Polygon(coords), classes))
+                            queue.append((shapely.Polygon(coords), set(classes)))
                 if is_closed_polygon:
                     el.getparent().remove(el)