Normal map rendering errors

[echadwick-artist]

Normal maps seem to be rendering incorrectly in the sample viewer.

NormalTangentTest

I noticed this when testing a new model for the glTF Sample Assets repo, which is made to test the rendering of normal maps which have been baked from high-resolution geometry into the UVs of lower-resolutions geometry:
KhronosGroup/glTF-Sample-Assets#88

The three cubes in the middle should be lit similarly to the cube at the top, but they are not.

Compare with how it is rendered in the Babylon.js Sandbox:
https://sandbox.babylonjs.com/

See also the ground-truth renderer Dassault Systèmes Enterprise PBR Pathtracer:
https://dassaultsystemes-technology.github.io/dspbr-pt/

[echadwick-dgg3d]

Thanks for the fix!