FBXLoader conditional animation parsing

[looeee]

FBXLoader currently creates a skeleton for all loaded models, whether or not they contain an animation. It then dumps everything (not just bones, but all meshes, groups, lights, cameras etc.) into the skeleton and uses that to link the animation.

As the comment mentions, this is quite a hacky way of doing things and should be rewritten, but in the meantime this at least solves the problem for files that don't contain any animation.

[jbaicoianu]

If this gets merged before #11603 and ends up putting it in conflict, I will be very sad :(

[looeee]

Note that this checks for 'Takes' in FBXTree to tell whether there is an animation.

Actual animation data is stored here

FBXTree.Objects.subNodes.AnimationCurveNode
FBXTree.Objects.subNodes.AnimationCurve
FBXTree.Objects.subNodes.AnimationLayer
FBXTree.Objects.subNodes.AnimationStack

While 'Takes' is used to specify multiple animation takes, so it may be neccesary to check for the presence of the above 4 nodes to know if there is animation.

However as far as I have been able to tell from the models I've tested, if there is an animation there will always be one take called "Take 001" specified, so the simpler test here should be sufficient.

[looeee]

@jbaicoianu these PRs don't overlap so no need to worry about that 😀

[mrdoob]

@jbaicoianu how should this PR look like if #11603 is merged?

[looeee]

Closing in favour of #12729