BufferGeometryUtils: Skip .userData in .mergeBufferGeometries()

[donmccurdy]

Currently .mergeBufferGeometries() adds a bunch of empty data to the .userData field, even if the original geometries did not use .userData themselves. This change ignores .userData if none of the objects have content. With this change we'll skip gathering .userData into an array. Since it is application-specific data, users can likely do something more appropriate with it easily. To reproduce the previous behavior, for example:

geometry.userData.mergeUserData = geometries.map( ( geometry ) => geometry.userData );

From https://threejs.org/examples/webgl_geometry_minecraft.html —

Before:

After:

[donmccurdy]

Hm, maybe we should just skip gathering the .userData entirely? I think this was added with #13241, because I wanted to avoid throwing away data during the loading process. We no longer use the function in GLTFLoader. And for users calling the function directly, it would be far simpler if we ask users to write:

geometry.userData.mergeUserData = geometries.map( ( geometry ) => geometry.userData );

So I don't think it's particularly relevant for BufferGeometryUtils to do this any more.

[Mugen87]

TBH, as a user I would expected that userData fields are honored by a merge operation, too.

[Mugen87]

On the other hand.....we don't know how users actually utilize the userData field on app level so the current merge might not be that useful.

Maybe it's indeed better to remove the exiting code. If someone complains, we can reconsider the code of this PR.

[donmccurdy]

Yeah, mostly I think it's hard to say what honoring .userData really means when it's application-specific data. It turns .mergedUserData into a sort of public API. I'd vote to remove this, and have updated the PR.