In #1839 we spec the behavior of reading from WebGPU canvases from other APIs, but we need to clarify whether compositingAlphaMode affects those.

1. If compositingAlphaMode is "opaque" does that mean e.g. drawImage reads opaque values?
2. If we add compositingAlphaMode "unpremultiplied" would that mean e.g. texImage2D sees it as an unpremultiplied source?

My initial feeling is that the answers to both should be yes. In that case we should also probably rename compositingAlphaMode to something like canvasAlphaMode.