We are being lax here, but the glTF specification just requires that "when the weights are stored using float component type, their linear sum SHOULD be as close as reasonably possible to 1.0 for a given vertex"
In particular weights > 1 and weight sums well below or above 1 can be observed in models exported by Blender if they aren't manually normalized.
These fail the glTF validator but Irrlicht normalizes weights itself so we can support them just fine.
The docs have been updated to recommend normalizing weights (as well as documenting the status of interpolation support).
Weights < 0, most of them close to 0, also occur. Consistent with Irrlicht, we ignore them, but we also raise a warning.
Also removes all animations but the first one from gltf_frog.gltf
to address the corresponding warning.
Catches some more possible exceptions (out of bounds, optional access)
which might be caused by a broken model to properly log them.
