Function selectVariant

Select a material variant by name on a loaded glTF asset.

Two-step operation:

1. Restores ALL variant-participating meshes to their original (default) materials.
2. Applies the selected variant's material overrides.

This ensures meshes without a mapping for the new variant revert to their defaults, even when switching between variants (e.g. Red → White).

Works both before and after addToScene():

• Before: materials are set directly (renderable built later).
• After: the property-setter intercept triggers renderable rebuild automatically.