This brings CollisionPolygon3D up to feature parity with its counterpart
CollisionShape3D. Closes#101414.
In addition, adding this feature fixes the issue that CollisionPolygon3D
would never be rendered in exported builds, even if Visible Collision Shapes
is enabled at runtime. Closes#101413.
This harmonizes the appearance of collision shapes and paths between
the editor and running project, in both 2D and 3D.
This means that in 3D, paths are now green and shapes are now cyan
instead of light blue.
