Preserve unknown gltf mesh extensions in GLTFLoader

[cstegel]

Description

This is needed for custom glTF extensions which handle extension data under each mesh entry in the glTF json.

Previously, only unknown extensions in the glTF json's meshes[].primitives[].extensions field were added to the populated mesh's userData.gltfExtensions field. It is also valid to add data under glTF json meshes[].extensions so those should be added to the resulting mesh, too.

[donmccurdy]

Thanks @cstegel! When a glTF mesh has multiple primitives, I'm not sure we should be duplicating the extensions across all of the resulting THREE.Mesh instances. How about something like this, later in the affected function?

if ( meshes.length === 1 ) {

	if ( meshDef.extensions ) addUnknownExtensionsToUserData( extensions, meshes[ 0 ], meshDef );

	return meshes[ 0 ];

}

const group = new Group();

if ( meshDef.extensions ) addUnknownExtensionsToUserData( extensions, group, meshDef );

...

[cstegel]

@donmccurdy sounds good! I didn't notice that multiple primitives would get put together in their own Group. Adding it to the Group seems fine to me. I've applied those changes now.

[donmccurdy]

Thanks @cstegel!