Change skinned mesh warning to only consider transformed parent nodes

[aaronfranke]

This PR replaces NODE_SKINNED_MESH_NON_ROOT with NODE_SKINNED_MESH_PARENT_TRANSFORMS to avoid false positives for valid models. Fixes #133

I believe this check more accurately reflects the purpose of the original check. It's not that parent-child relationships entirely are considered invalid, rather that the transform aspect of skinned mesh nodes is ignored with respect to the visible mesh. The test model ignored_parent_transform.gltf is already set up this way, testing with a parent translated by [1, 2, 3]. There are valid use cases for inserting a skinned mesh into the hierarchy, for purposes like visibility, but we only need to validate for transforms.

[CLAassistant]

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[lexaknyazev]

The original warning also implicitly flags cases when the static transform is identity but the parent nodes are animated. This PR would reduce such coverage.

We could instead reduce the severity of the original message and add a new warning that only flags static transforms (as these are clearly problematic).

[aaronfranke]

Done, the original message now has reduced severity, and a different message replaces it when the parent is transformed.

[lexaknyazev]

Please don't replace the original message when the new message is added. Seeing both messages upfront would be a better UX than seeing the parent transforms warning, fixing it, and discovering that the asset still has issues.