Add layer vs non-layer example and layer hit test example

[felixtrz]

This PR adds:

1. the layer vs non-layer example that shows the quality difference of using a quad layer to render an image vs rendering the same image in projection layer as a simple texture.

2. the layer hit test example that shows how to do hit test with quad layers.

   @toji

[toji]

I apologize. I reported that the samples were causing a crash but apparently is something wrong with running any WebXR content on my specific device. I'm in the process of debugging it now, and will give this PR a proper review once that's solved.