- For Cell#shapes, LayerInfo is an accepted argument now
- If the layer does not exist, it is created (non-const version)
or an error is raised (const version)
- Cell#clear also accepts a LayerInfo object
- Layout#clear and Layout#delete_layer also accept
a LayerInfo parameter
Rationale for supporting the clear and delete_layer methods:
These cases do nothing if the layer does not exist, so there
is a benefit of using them: it is not required to check first
if the layer exists.