proposal: OMI_animation_default

[yankscally]

To be totally thorough, I will start making soft proposals here in the issues section of omigroup/gltf-extensions. I want to have some long form discussion before even trying to write any draft specifications.

My first proposal is a simple one:
default animations.

When opening a glTF file, you might expect to have an animation play automatically, or you might want a game engine to load a specific animation and play that when the level loads.

In this video I use a default animation extension to move multiple platforms:
video

I simply put a default animation flag in extensions/VENDOR_animation_default/defaultAnimation/<animation>

This lets the importing application know what the default animation is.