Normals on font geometry in TransmissionThinwallTestGrid.gltf modeled in geometry plane

Visualizing the world space shading normals inside the [TransmissionThinwallTestGrid.gltf](https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/TransmissionThinwallTestGrid) model shows that all font geometry has normals which are not consistent with the background planes behind them (color = N * 0.5f + 0.5f):
The light green means the normals are (0, 1, 0) instead of (0, 0, 1) like on the background planes.

This can result in unexpected lighting results because the core material default for dielectric specular reflections is not black and would handle this view as Fresnel near 90 degrees which should produce unwanted reflections.

Inspect TransmissionThinwallTestGrid.gltf and compare the font geometry normals with the background plane normals in a viewer that displays world-space normals. Determine how the asset's font geometry was modeled and correct the normals so they match the background planes. Done means the font and background normals are consistent and the reported unwanted Fresnel lighting behavior is no longer present.

Written by the indexing model from the issue text.