Thanks to a refactor, `Resource::duplicate_for_local_scene()` and `Resource::duplicate()` are now both users of the same, parametrized, implementation.
`Resource::duplicate()` now honors deepness in a more consistent and predictable fashion. `Resource::duplicate_deep()` is added (instead of just adding a parameter to the former, for compatibility needs).
The behavior after this change is as follows:
- Deep (`deep=true`, formerly `subresources=true`):
- Previously, only resources found as direct property values of the one to copy would be, recursively, duplicated.
- Now, in addition, arrays and dictionaries are walked so the copy is truly deep, and only local subresources found across are copied.
- Previously, subresources would be duplicated as many times as being referenced throughout the main resource.
- Now, each subresource is only duplicated once and from that point, a referenced to that single copy is used. That's the enhanced behavior that `duplicate_for_local_scene()` already featured.
- The behavior with respect to packed arrays is still duplication.
- Formerly, arrays and dictionaries were recursive duplicated, with resources ignored.
- Now, arrays and dictionaries are recursive duplicated, with resources duplicated.
- When doing it through `duplicate_deep()`, there's a` deep_subresources_mode` parameter, with various possibilites to control if no resources are duplicated (so arrays, etc. are, but keeping referencing the originals), if only the internal ones are (resources with no non-local path, the default), or if all of them are. The default is to copy every subresource, just like `duplicate(true)`.
- Not deep (`deep=false`, formerly `subresources=false`): <a name="resource-shallow"></a>
- Previously, the first level of resources found as direct property values would be duplicated unconditionally. Packed arrays, arrays and dictionaries were non-recursively duplicated.
- Now, no subresource found at any level in any form will be duplicated, but the original reference kept instead. Packed arrays, arrays and dictionaries are referenced, not duplicated at all.
- Now, resources found as values of always-duplicate properties are duplicated, recursively or not matching what was requested for the root call.
This commit also changes what's the virtual method to override to customize the duplication (now it's the protected `_duplicate()` instead of the public `duplicate()`).
