Skinned glTF models with armature scaling don't work well

Two example models are `char.glb` from #8175, and `bee.glb` from #8285.

In both cases, the optional property `skeleton` was omitted, causing a crash in versions of Cesium up until now. The crash is fixed in PR #8756.

Also in both cases, the root node has an applied scaling factor of about `0.01`. This causes the model to remain invisible (even after the crash is fixed).

It looks like possibly the bounding sphere calculations are incorrect, or a mismatch for what the root scaling is doing to the bones and rigging. In this sample model, the above mentioned `char.glb` has been modified, such that the root scale has changed from `0.01` to `0.5`:

[char_halfScale.zip](https://github.com/CesiumGS/cesium/files/4483695/char_halfScale.zip)

Loading this model into the Sandcastle 3D Models demo, it looks like the camera focuses on a spot above the character's head. The bounding sphere is apparently not centered on the model, and likely not sized correctly.

Reproduce the issue with char.glb and bee.glb in the Sandcastle 3D Models demo, including the modified char_halfScale model. Read the changes in PR #8756 first, then investigate the bounding sphere and root scaling behavior for skinned models. Done means the models load without a crash, remain visible, and the camera focuses on a correctly centered and sized bounding sphere.

Written by the indexing model from the issue text.