GLTFLoader: Pre-compute geometry bounds using attribute min/max

[zeux]

Instead of relying on three.js to compute bounding box/sphere, we now
compute them from attribute min/max specified in glTF file. These are
mandatory as per glTF spec.

This allows us to save time after loading the object - bounding sphere
is used for culling unconditionally.

Note that since glTF data doesn't directly specify bounding sphere data,
the bounding sphere is slightly larger because it's computed from min/max
values.

Fixes #17937

[mrdoob]

Thanks!

[Mugen87]

It seems this change broke the VRM example:

https://threejs.org/examples/webgl_loader_vrm

The min and max accessor values are undefined und thus a runtime error occurs.

/cc @takahirox

[zeux]

@Mugen87 Thanks for the heads up! For what it's worth, I intentionally did not put any checks for min/max accessor presence because glTF specification mandates their presence on position data for both base mesh and morph targets.

Indeed, if I rename the file in question to .glb extension and run it through glTF validator, I get 563 errors, around half are in the form of

/meshes/2/primitives/0/targets/2/POSITION: accessor.min and accessor.max must be defined for POSITION attribute accessor.

We can obviously add handling for this to accommodate non-compliant assets but it seems like it would be better to fix the model in question, as my understanding is that VRM models need to be glTF 2.0 compliant?

[Mugen87]

I guess @takahirox can help here since the loader and example were introduced via #13877.

Maybe that's a good opportunity to update the model itself since it was a bit controversial^^ #13877 (comment)

[zeux]

Sounds good. If we need to support this in the loader it's simple, just need an early-out similar to the one we have for POSITION.