ACCESSOR_WEIGHTS_NON_NORMALIZED is no longer consistent with the specification

[tigercat2000]

Hi, in the following commits:

KhronosGroup/glTF@0a61cf8
KhronosGroup/glTF@94ed681#diff-ca628c11679a8c4c0c4031440c7c1cda27c8896ebc87c3bd1e8640efc8942d99R1451

The MUST be normalized to have a linear sum of 1.0 requirement was morphed into a SHOULD be as close as reasonably possible to '1.0' for a given vertex. requirement.

However, glTF-Validator still marks this as an error rather than a warning.

I'm not sure if this is a problem with the specification or validator. However, as the specification is no longer in pre-release, I'm assuming this is the way that it shall stand.

To explain, I believe this might be an error with the specification as it appears that KhronosGroup/glTF@94ed681 may have seen the intermediate explanation with "should" in the paragraph and upgraded it to BCP 14, whereas KhronosGroup/glTF@0a61cf8 stated it was not a breaking change and was only meant to elaborate.