fix invalid bone weights

[robtfm]

Objective

when a mesh uses zero for all bone weights, vertices end up in the middle of the screen.

Solution

we can address this by explicitly setting the first bone weight to 1 when the weights are given as zero. this is the approach taken by unity (although that also sets the bone index to zero) and three.js, and likely other engines.

Alternatives

it does add a bit of overhead, and users can always fix this themselves, though it's a bit awkward particularly with gltfs.

(note - this is for work so my sme status shouldn't apply)