Fix new skinning issues

[lexaknyazev]

After updating the validator there are a few new skinning issues in the generated models (see reports for details):

• Animation_Skin_02
  • [ ] NODE_SKINNED_MESH_NON_ROOT
  • [ ] NODE_SKINNED_MESH_LOCAL_TRANSFORMS
• Animation_Skin_06
  • [ ] SKIN_NO_COMMON_ROOT
• Animation_Skin_07 and Animation_Skin_10 (see #559)
  • [x] ACCESSOR_JOINTS_USED_ZERO_WEIGHT

[lexaknyazev]

The description of Animation_Skin_02 says:

skinA where the skinned node has a transform and a parent node with a transform. Both transforms should be ignored.

So it seems that the warnings should remain intact, i.e. we won't fix them.

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The description of Animation_Skin_06 says:

skinA where joint1 is a root node and not a child of joint0.

This is clearly a violation of the "common root" requirement, so this model should either be fixed or removed.