bad comment
initialize children collection in init instead
removed group behavior

I didn’t mean for PolygonNode to have this behavior to begin with. TODO
need to put the child abbox code somewhere else.
added group behavior

Moved group behavior from PolygonNode to this class as I should have
done in the beginning.
added null check and corrected type check.

The cascade was checking the wrong object type. It was checking itself
instead Color4Mixin
whitespace
now inherits from Color4Mixin

Nodes should implement Color4Mixin to properly participate in
animations and cascading.
updated diagram
fixed cascading functionality.

Cascading was checking the wrong children type