[1.0] VRM-1.0 の targetNames を Mesh.extras のみに修正

[ousttrue]

#1739

targetName

下記参照のこと。

https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#meshes

A significant number of authoring and client implementations associate names with morph targets. While the glTF 2.0 specification currently does not provide a way to specify names, most tools use an array of strings, mesh.extras.targetNames, for this purpose. The targetNames array and all primitive targets arrays must have the same length.

歴史的事情により、当初 primitive.extras.targetNames を使っていた。

KhronosGroup/glTF#1036

[Santarh]

よし