This changes the WebGL and desktop glTF viewers so that they always
call `updateBoneMatrices`, even when there are 0 animations. Note
that our Android sample was already doing this correctly.
When combined with #5301, this fixes#5299, although users would need to
use the `recomputeBoundingBoxes` feature in gltfio (`-r` in the desktop
viewer) for this model to be scaled to fill the viewport, since its
embedded bounds are quite large.
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