Textures for sponza scene are missing alpha channels

I've been wanting to add transparency to the ray tracing extended sample, so that plants, chains, etc. would be rendered with proper transparency instead of looking like solid objects.

But it seems that the sponza scene we are including as an asset doesn't have an alpha channel for transparent objects, making it impossible to properly render the scene.

We should fix this and add proper alpha channels.

Inspect the included Sponza scene textures and how the ray tracing extended sample references them. Identify the textures used by transparent objects and update them with alpha channels; done means the scene assets preserve transparency information for plants, chains, and similar objects.

Written by the indexing model from the issue text.