Add missing glClear call to layer samples

[idrisshah]

Quad layer samples are broken because of missing glClear on Magic Leap system. These samples miss a glClear call before every frame rendered.

** This patch is funded by Magic Leap

[cabanier]

This clear isn't needed. WebXR Textures are supposed to be cleared when you get them.
See https://immersive-web.github.io/layers/#xropaquetextures

I agree that this is not obvious from an implementor's point of view. It was hard to implement in a performant way

[idrisshah]

Thanks for this clarification. I missed that part. In that case, there are some samples that still have glClear calls like: https://immersive-web.github.io/webxr-samples/layers-samples/proj-layer.html. Should those be cleaned up for consistency?

[cabanier]

Thanks for this clarification. I missed that part. In that case, there are some samples that still have glClear calls like: https://immersive-web.github.io/webxr-samples/layers-samples/proj-layer.html. Should those be cleaned up for consistency?

yes, that would be good