WebGPU: Set compositingAlphaMode

[sunag]

Description

Fix alert:

This contribution is funded by Google via Igalia.

[mrdoob]

Thanks!

[WestLangley]

I think compositingAlphaMode: 'premultiplied' is the correct choice.

We should have a test case with a CSS background color. I'm thinking clearAlpha should default to zero, too.

[mrdoob]

@Kangz What do you recommend?

[Kangz]

I'm not 100% sure. I think compositingAlphaMode: 'premultiplied' is more efficient on macOS but I don't know about other OSes, and if that behavior is what Three would want.

[WestLangley]

The reasons to set 'premultiplied' are:

1. with opaque, a transparent background is not possible, and
2. the NormalBlending function assumes the drawing buffer is premultiplied.