Textures for sponza scene are missing alpha channels

[SaschaWillems]

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.