Handling of missing geomprops in GLSL backend

When loading the following document:
```xml

```

MaterialXView displays an error:
"Geometry buffer could not be retrieved for binding: i_geomprop_missing. Index 0".

I don't think this is correct behaviour. The `` node has a `default` input which's value should be used instead.

Start by loading the supplied MaterialX document in MaterialXView and reproduce the GLSL backend error for the missing geometry property. Trace how the geompropvalue node's default input is handled, then verify that the default color is used without the geometry-buffer error.

Written by the indexing model from the issue text.