Mesh data validation

Validator should be able to detect technical issues with mesh data such as degenerate or incomplete primitives. As of 55a960559a6b4cf4f9ad0d598443477943b792fc, only degenerate triangles are reported.

Next possible features (in no particular order):
- Index-based detection of degenerate primitives (all types except POINTS).
- Index-based detection of duplicate primitives. It must be clockwise-aware.
- Positions-based detection of duplicate/degenerate primitives. Must be optional because such validation could be slow and generate false positives (e.g., for meshes intended for further runtime processing).
- Detection of duplicate vertices (i.e., exactly same values across all attributes).
- Normals and tangents "correctness" (how to rigorously define this for all meshes?).

Thoughts?
@pjcozzi @bghgary @emackey @javagl @UX3D-nopper

Start by reviewing the existing degenerate-triangle reporting mentioned in the issue. The issue lists several possible validation features without selecting one or naming files, entry points, tests, or a completion condition, so scope and expected behavior must be settled before implementation.

Written by the indexing model from the issue text.