What do we want to test instancing with

[stevk]

Below is the list so far.

• Nodes pointing to the same mesh
• Nodes pointing to the same skin
• Primitives pointing to the same material
• Materials pointing to the same texture

[bghgary]

That's a good start.

Here are some more:

• Primitive indices/attributes pointing to the same accessors
• Accessors pointing to the same buffer views
• Buffer views pointing to the same buffers
• Textures pointing to the same images

When we do animations, etc.:

• Animation channels pointing to the same samplers
• Samplers pointing to the same accessors
• Morph target attributes pointing to the same accessors
• Skins pointing to the same skeleton
• Skins pointing to the same inverseBindMatrices (not sure about this one)

There are probably more.

[stevk]

@kcoley could you mark which areas were covered by #462 and close if they've all been addressed.

[stevk]

There are models using instancing in several modelgroups, but there should be one modelgroup specifically showcasing instancing, to make these easier to find.

[stevk]

Morph is NYI.