Import tangents

[david-lively]

Description

Previously, the glTF importer would ignore tangents in the model. This PR ensure that tangents in the vertex stream are properly imported and transferred to the relevant vertex buffer.

(This depends on #611 so merge that first.)

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

Load a glTF that includes tangents, such as the glTF Normal Tangent Mirror Test.
In Unity's Render Debugger, activate the Tangent material override and verify that tangents are included.
By default, Unity provides (1,0,0) for tangents if they are missing. The tangent vertex attribute override should show something other than (1,0,0) .