Fix a couple issues with glTF tangent processing.

[emackey]

Recently I created a new model, NormalTangentMirrorTest to test what happens in a rendering engine when a glTF supplies tangent vectors. A previous model called NormalTangentTest was testing an engine's ability to calculate missing tangent vectors.

Cesium does a great job of calculating any missing tangent vectors, but unfortunately, when the vectors are supplied in the model, things were very broken. Looking at the code, I found these issues:

• Cesium was treating 'TANGENT' as VEC3 when the glTF spec says it's VEC4.
• Cesium was multiplying tangents by u_modelViewMatrix instead of u_normalMatrix.
• Cesium was not taking the w component of the tangent into account.

The glTF spec has this to say about using the w component of supplied tangent vectors:

Client implementations should compute the bitangent by taking the cross product of the normal and tangent xyz vectors and multiplying against the w component of the tangent: bitangent = cross(normal, tangent.xyz) * tangent.w

On master, my new tangent test model looks like this:

On this branch, with the above-mentioned fixes applied, things look correct:

[cesium-concierge]

@emackey, thanks for the pull request! Maintainers, we have a signed CLA from @emackey, so you can review this at any time.

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[mramato]

Thanks @emackey, did you open a corresponding gltf-pipeline request? This is third party code from that repository's 2.0 branch.

[emackey]

Yes, it's: CesiumGS/gltf-pipeline#350

[lilleyse]

Thanks for the fix!