Clear opaque framebuffer every frame and when first created

[takahirox]

https://www.w3.org/TR/webxr/#xrwebgllayer-interface

The buffers attached to an opaque framebuffer MUST be cleared to the values in the table below when first created, or prior to the processing of each XR animation frame. This is identical to the behavior of the WebGL context’s default framebuffer.

For example, WebXR samples AR hit test example expects it so doesn't clear in their side.

[takahirox]

Closing in favor of #82