[hdSt] Incorrect MaterialX shading without dome light

[pablode]

Description of Issue

Rendering MaterialX materials in hdStorm without a dome light results in rendering artifacts:

The background color should be used as a fallback for environment reflections if no dome light is active. One way of achieving this could be by creating a 1x1 dome light texture.

I've attached a sample model from Cesium, licensed under CC-BY, converted to USD+MaterialX.

Steps to Reproduce

1. open this model in usdview: CesiumMilkTruck.zip
2. deactivate the 'UsdPreviewSurface' prim
3. observe rendering artifacts
4. turn on dome light, see the artifacts disappear

System Information (OS, Hardware)

Windows, RTX 2060

Build Flags

OIIO and MaterialX

[jilliene]

Filed as internal issue #USD-7383