Prune fails to clean up accessors referenced by non-root properties

Resources like Accessors may be considered 'in use' by prune() if they're referenced by unused primitives or primitive targets. Because no references exist to the primitives or primitive targets, we won't find those and dispose them when iterating over meshes, and the empty accessors are written to disk despite no real use.

The other non-root entity types in the core spec are TextureInfo, AnimationSampler, and AnimationChannel. TextureInfo won't have this issue, but the other two might. Conceivably this could also happen with extensions, but prune() doesn't currently attempt to identify unused extension resources, so that's out of scope for now.

Perhaps it would be better to do a formal tree-shake, iterating all nodes in the graph, and pruning those we can't trace to the Root? This might simplify the current implementation of prune() as well.

Start at the prune() implementation and trace how references are collected while iterating meshes, then inspect handling for Accessors, primitive targets, AnimationSampler, and AnimationChannel. Done means unused non-root resources are removed without affecting resources reachable from Root; extensions remain out of scope.

Written by the indexing model from the issue text.