Web Viewer: not all inputs are displayed

I was looking at the web version of MaterialXView: https://academysoftwarefoundation.github.io/MaterialX/
and noticed that the inputs displayed in the UI don't match the actually expected inputs.

Some examples:

**standard_surface_marble_solid.mtlx**

Expected would be two additional color slots:
```xml





```

**myFile.mtlx**

Expected would be two inputs, `base_color` and `specular_roughness`:
```xml

```

It looks like the UI generation code has some heuristics in place, e.g. checks if there's "standard_surface/base" before adding "standard_surface/base_color" to the UI, but I'm not sure if that's all there is to it.

Start at the MaterialXView UI generation code and inspect the heuristic around `standard_surface/base` and `standard_surface/base_color`. Compare its displayed inputs with the `standard_surface_marble_solid.mtlx` and `myFile.mtlx` examples; done means all expected connected inputs appear in the web viewer.

Written by the indexing model from the issue text.