Add tap to swap material feature

[benmorrow]

No description provided.

[asavihay]

Thanks @benmorrow!
My goal was to keep an example as basic as possible with no extra logic. I thought that this is a good way to pass this knowledge.

Do you think the tapping will add to the next person that will review this code ? I have no objection merging :)

[benmorrow]

I thought it might help to understand what the materials look like so you don't have to change the value four times in code and run four times to see each separate material. I know that's the first thing I wanted to check out when I ran the app: "What does this physical rendering thing look like?"