Model compute world bounding sphere

NOTE: This is a pull request into the `gltf` branch.

Its possible that max{length(sphere[i].center - center) + sphere[i].radius} > max{length(sphere[i].center - center)} + sphere[i].radius. For example, if the model has two bounding spheres that have centers an equal distance apart, the first one has a radius 2m and the second has a radius 2km, then we should choose the second instead of the first.