When EEVEE next removed the Alpha Threshold option a node graph setup
was used instead. Based on a USD test asset[1] the original code was
designed around only using the Image A channel during import to wire
up the nodes. However, the other channels (R, G, or B) are just as
valid.
We now handle this by wiring up the Separate RGB node, that is already
inserted for the scenario in general, to the shader nodes handling the
threshold calculation. Also adds additional test coverage for both
export and import as well.
The Round and LessThan+OneMinus networks remain the only shapes that
will be recognized for the processing.
[1] https://github.com/usd-wg/assets/tree/main/test_assets/AlphaBlendModeTest
Pull Request: https://projects.blender.org/blender/blender/pulls/130346
39e96d9c0f