GLTFExporter: Fix handling of shared buffer attributes with morph data. (#24090)
GLTFExporter: Fix handling of shared buffer attributes with morph data. (#24090)

* GLTFExporter: Fix handling of shared buffer attribtues with morph data.

* GLTFExporter: Simplify code.

* GLTFExporter: Clean up.
mrdoob/three.js,donmccurdy/three.js,aardgoose/three.js,mrdoob/three.js,WestLangley/three.js,Liuer/three.js,gero3/three.js,Liuer/three.js,06wj/three.js,makc/three.js.fork,gero3/three.js,WestLangley/three.js,makc/three.js.fork,donmccurdy/three.js,06wj/three.js,aardgoose/three.js