Fix unused joint indices

[lexaknyazev]

This PR fixes ACCESSOR_JOINTS_USED_ZERO_WEIGHT warning for Animation_Skin_07 and Animation_Skin_10.

Similar code in CreateJointsAndWeightsForMultipleRoots was not edited because it's currently unused.

[bghgary]

I'm not sure I understand this skin warning in the validator. Are we saying we must use joint index 0 if the weight is 0?

[lexaknyazev]

Yes, the validator follows this statement from the spec:

Unused joint values (i.e. joints with a weight of zero) should be set to zero.

The reasoning is:

1. There could be cases when a vertex has less than 4 influences.
2. Yet, the accessor is VEC4, so something must be written to missing joint indices.
3. Given that we decided to not introduce a "special" index value (like 255 or 65535) for unused influences, there's no way to distinguish between truly unused (as if there are only 3 joints total) and "normal" zero-weight (like in this example) influences.
4. Therefore, all zero-weight influences should have zero joint index.

[bghgary]

Yeah, I vaguely recall this discussion now. Thanks!