Support collapsing of visual links

[Garrett Johnson]

The collada format affords storing a tree of visual data, meaning that not every visual link needs to export its own geometry. There should be some way to notify the exporter that a provided mesh recursively includes all child geometry so that subsequent links don't need to be traversed.

Approach

• Run every link through the mesh function, even it if has no geometry
• Returning null means no visual node / geometry
• This would remove the ability to cache and reuse processed meshes.

Questions

• Is it possible to run through the model and then merge any visual nodes that are just fixed?

[Garrett Johnson]

This is supported flexibly enough by isLeaf