Fixing the BufferView "validation" issue

[vpenades]

Recently I've been reported several issues related to BufferView not being defined when loading a model, often related to sparse accessors, but not limited to them.

At first sight it may look an oversight on my side with a solution as simple as removing these checks, but that would have been a disaster.

That validation prevents bigger problems to happen down the road because a key feature that is required.

As per the specification: 'When accessor.bufferView is undefined, the sparse accessor is initialized as an array of zeros of size'

This means that it is not enough to simply skip the validation: a placeholder buffer needs to be set in place an filled with zeros.

And this can be done in a number of ways, but there's a catch: the Accessor's data is exposed via a number of vertex attribute codecs that help encode and decode the bytes to meaningful values, and these codecs are designed to have read and write access. this means that a vertex attribute codec could be used to fill the placeholder buffer with non zero values, opening the door for incredibly hard to find errors.

So, in order to fix this apparently "simple" issue, it will be neccesary to completely redesign the BufferView, Accessor and vertex attribute codecs to discriminate between "ReadOnly" and "ReadWrite" buffers.

This is the next thing I'm going to work with, it will take a while, and it has the potential for API breaking changes.

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