Implement (Generate) Sparse Accessors

[zellski]

Especially when outputting animation data, leaning on sparse accessors could likely save a fair bit of space in the output file.

If we end up Draco-compression animations in the future, the value of sparse accessors goes down; long runs of zeros largely vanish in compressed form. Even in that situation, though, there are time- and memory-saving benefits on the client.

[zellski]

With morph targets in place, the priority of this has increased. Their storage in full form can be very inefficient.

[zellski]

With mrdoob/three.js#12596 done, I'm bumping this to 1.0 requirement. Morph targets are insanely inefficient without sparse accessors.

[jiamingfeng]

Any update on this one? Is there an option to keep the original animation instead of baking it every frame?

[zellski]

Afraid not. If you have resources to help, pull requests are welcome.