It's great to see how the design nicely fermented over the three and
half (!!) years. This is a completely opt-in feature for all plugins
involved, so if the plugin decides to not do anything about it, it
doesn't have to. The actual enforcements will come later.

Because there's no format ever that would support zero-copy everything
(except the upcoming Magnum blobs, hah!), it makes no sense to force
zero-copy globally. Instead, zero-copy can be forced just for particular
data of interest. For example a glTF can zero-copy-import mesh and
animation data, but not always image data (except if it embeds an
uncompressed KTX), and never scene or material data, which are stored in
the textual JSON.

Meant to be used from within plugin implementations to avoid copies in
the delegated importers.

TODO: the heck, why can't I call a protected function from a subclass
   but another instance?
TODO: figure out a way to test this
TODO: update docs and behavior for the ZeroCopy flag
TODO: adapt AnySceneImporter also