fix(skeleton): skip unused vertex attributes
fix(skeleton): skip unused vertex attributes

If a vertex attribute is unused, it may be optimized out completely and given an ID of -1.  This
change avoids the WebGL warnings that result from attempting to enable and bind to vertex attribute
- 1.

google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer