Converge dual-source blending with the WebGPU standard

[ErichDonGubler]

https://www.w3.org/TR/webgpu/#dom-gpufeaturename-dual-source-blending

Firefox downstream tracking: bug 1924328

[teoxoy]

We already support dual-source blending (https://github.com/gfx-rs/wgpu/issues/1804) but I think our validation is not 1:1 with the spec.

[ErichDonGubler]

@teoxoy: Ah, yes. I've renamed the issue to note that the scope is to converge with the spec.

[jimblandy]

So the work here is:

• Figure out what the spec says
• Figure out what we're doing wrong, exactly

Teo suspects that our validation is too strict.

We are using the wrong names for things.

We don't have the enable implemented in the WGSL front end.

There are probably API bits too.

[jimblandy]

Also, we're probably exposing this even when the feature isn't enabled.

[Wumpf]

started looking into this because I need it for a toy project :) Expect my first Naga PR soon 😄