Fix #1626 Set skinned mesh at root

[julienduroure]

Fix #1626
Set skinned mesh at root

Note : Need to be fully tested

[scurest]

Even if the transform is not needed for the skinned mesh, there may be child objects that need the transform. The importer handles this by possibly creating a new node to attach the skinned mesh to.

Example

1. Import CesiumMan.
2. Parent a cube to the CesiumMan mesh.
3. Drag the armature so its off-center.
4. Export and view in viewer. On this branch, the cube is at the origin instead of off-center with the CesiumMan mesh.

[donmccurdy]

Copying from #1626 (comment) —

Understanding that applying transforms to the skinned mesh node (as opposed to the armature / joints) does not have an effect in glTF ... don't we still need to apply those transforms somewhere, if they were contributing to the final position of the vertices in Blender? Applying the transform to the inverse bind matrices might be the best approach if that's the case.

^See @scurest's #1626 (comment), my comment doesn't need to block the PR.

[julienduroure]

Trying a fix to the case described by @scurest
by resetting not only the TRS of glTF mesh, but also the internal world matrix that is used for children calculation

[julienduroure]

So, any other cases that breaks or are not solved?
Don't hesitate to tell me

[julienduroure]

Note: This breaks driver detection for now