Enhance validation report with specifics about geometry issues

[lexaknyazev]

trafficstars

See https://github.com/AnalyticalGraphicsInc/gltf-vscode/issues/43#issuecomment-342311369 for context.

[lexaknyazev]

@emackey @bghgary With new debugging capabilities in VS Code plugin, can we visualize outstanding geometry issues?

Low-hanging fruit seems to be ACCESSOR_INDEX_TRIANGLE_DEGENERATE. Validation report can be easily extended with code-specific data value:

{
    "code": "ACCESSOR_INDEX_TRIANGLE_DEGENERATE",
    "message": "Indices accessor contains 2 degenerate triangles.",
    "severity": 2,
    "pointer": "/accessors/0",
    "data": [578, 67]
}

There's an issue, though. The validator calls out accessors directly, without linking them back to meshes. Should we enhance this somehow?

[bghgary]

accessors directly, without linking them back to meshes

In order to show the triangle, there certainly needs to be info about which mesh it is. Ideally, it should be the mesh primitive json pointer and what triangle it is. We can also possibly select the triangles in question in the mesh primitive view.

[lexaknyazev]

With all glTF indirection, data could be shared like:

• a mesh having 2+ primitives using the same index buffer;
• 2+ meshes with primitives using the same index buffer.

How should the validation output look like for these?