Better morph target support

[mgsx-dev]

As per specification :

The number of morph targets is not limited. Client implementations SHOULD support at least eight morphed attributes. This means that they SHOULD support eight morph targets when each morph target has one attribute, four morph targets where each morph target has two attributes, or two morph targets where each morph target has three or four attributes.

For assets that contain a higher number of morphed attributes, client implementations MAY choose to only use the eight attributes of the morph targets with the highest weights.

gdx-gltf currently only supports up to 8 morph targets, and it crash when using more. So it needs to have at least a clear error message.