Add option to gltf_viewer to play all animations

[bejado]

glTF files exported from Blender export multiple animations, separated per object. This adds a new "Play All" option to simultaneously play all the animations present in the glTF file:

[prideout]

I actually think we shouldn't merge this. In fact I originally used checkboxes but this turned out to be wrong. According to the spec, best practice is:

...each animation is self contained as an action. For example, "Walk" and "Run" animations might each contain multiple channels targeting a model's various bones. The client implementation may choose when to play any of the available animations.

The Blender exporter might be doing something wrong. It often does.

(For further details see KhronosGroup/glTF#1730)

[bejado]

@prideout Interesting point. Yeah, I'm not sure why Blender chooses to do it this way. Perhaps the issue is better addressed there.

[donmccurdy]

Blender's animations (Actions, or NLA Tracks) are associated with a single object. Frustratingly, there is no mechanism in Blender to specify:

1. [Run, Walk, Idle] animations are meant for Character X, not Y.
2. [A, B, and C] animations are meant to be played on objects X, Y, and Z simultaneously.

As you can imagine, that creates challenges both exporting and importing. See KhronosGroup/glTF-Blender-IO#671.

We've gone through a few iterations in the Blender exporter, and landed on the behavior documented here as a solution for users who need multiple distinct animations.

The exporter would benefit from another option, to export all active actions as a single (global) glTF animation, but that has been lower priority, to date. Feel free to file an issue if that sounds like what you need!