Invalid skin weights on several skinned models.

[vpenades]

I've run gltf validator against all the sample models and I've identified some with invalid skin weights.

The error type is: Weights accessor elements (at indices X-X) have non-normalized sum.

The affected models are:

• CesiumMan
• Monster
• RiggedSimple
• RiggedFigure

The reported errors:

CesiumMan.glb
Errors: 210, Warnings: 1, Infos: 0, Hints: 0
Time: 24ms

	Errors:
		/meshes/0/primitives/0/attributes/WEIGHTS_0: Weights accessor elements (at indices 0..3) have non-normalized sum: 0.6854066252708435.
		/meshes/0/primitives/0/attributes/WEIGHTS_0: Weights accessor elements (at indices 4..7) have non-normalized sum: 0.5730400085449219.
		/meshes/0/primitives/0/attributes/WEIGHTS_0: Weights accessor elements (at indices 8..11) have non-normalized sum: 0.7170813679695129.
		/meshes/0/primitives/0/attributes/WEIGHTS_0: Weights accessor elements (at indices 12..15) have non-normalized sum: 0.5980731844902039.
		/meshes/0/primitives/0/attributes/WEIGHTS_0: Weights accessor elements (at indices 680..683) have non-normalized sum: 0.9898213148117065.
		/meshes/0/primitives/0/attributes/WEIGHTS_0: Weights accessor elements (at indices 688..691) have non-normalized sum: 0.9684941172599792.
		/meshes/0/primitives/0/attributes/WEIGHTS_0: Weights accessor elements (at indices 692..695) have non-normalized sum: 0.9564582705497742.
		...

[lexaknyazev]

See #245 (comment)

/cc @emackey