Warn about constant animation sampler output accessor values?

This is similar to https://github.com/KhronosGroup/glTF-Validator/issues/184 in some ways, but the differences in the technical details and implementation may justify bringing this up as an own issue:

Should there be a warning (or at least, a hint) when accessors that are used as `animations[a].samplers[s].output` have _constant_ values?

(Or are there any cases where it makes sense to export such constant values? One could probably think about extensions that might need this data or so, but from the core spec perspective, this does not seem to make sense...)

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(Apparently, some generators _always_ export _all_ properties (`rotation`, `translation`, `scale`) as animated properties as soon as _one_ property is animated, and ... apparently, sometimes even when no property is animated at all 😕 - of course, one could shrug this off and say "Yeah, then that's just a bad asset generator". But the implications on memory and performance may be _significant_, so I think that this should be strongly discouraged).

Start by comparing this proposal with glTF-Validator issue #184 and reviewing the core specification's treatment of animation sampler outputs. Determine whether constant output accessor values should produce a warning or hint, and define the relevant exceptions and validation behavior before implementation.

Written by the indexing model from the issue text.