In the past there was an API gotcha because users had to "get" the
animator before releasing the glTF source data. This could have been
surprising because it was a getter method, not a factory method.
This was due to overeager optimization on my part, I wanted to avoid
animator overhead for non-animated models, when in fact it has very
little overhead.
Moreover, the animator is conceivably useful even when there are no
pre-supplied animations (e.g. for applying skins), so let's just create
it unconditionally.
Motivated by #5299.
dc60f57a9a