2D Skeletal Animation

## What problem does this solve or what need does it fill?

For 2D games that want to have configurable characters with body parts, it's extremely verbose to create animation assets. Using sprite sheet animation only, the application suffers from art workflow, sheer raw amount of animation atlas, fragile parsing mechanism, and some runtime dynamic atlas building.

This is one evaluation point for choosing Bevy for our product's production use.

## What solution would you like?

A 2D skeletal animation system where body parts are designed to be attached to certain bones. This will leads to a simpler art workflow, skeletal animation to replace sprite sheets, and no more fragile parsing.

Performance impact on this could be more significant than sprite sheet animation. A good benchmark example of such feature would be nice to have, so that it's up to the dev team to decide whether to use this feature.

The solution you propose for the problem presented.

## What alternative(s) have you considered?

[Spine](http://esotericsoftware.com/) is a good solution out there, but Rust is lacking up to date spine runtime library.