Allow fragment stage to have more outputs than the pipeline has

[jinleili]

Connections
Follow #2469
WebGPU Spec: https://gpuweb.github.io/gpuweb/#fragment-state

Testing
Tested on metal, gl, vulkan, dx12 backends.
wasm target can not running on Chrome Canary because Dawn's validation failure:

Color target has no corresponding fragment stage output but writeMask (ColorWriteMask::(Red|Green|Blue|Alpha)) is not zero.
 - While validating targets[0].
 - While validating fragment state.
 - While calling [Device].CreateRenderPipeline([RenderPipelineDescriptor]).
localhost/:1 [Invalid RenderPipeline] is invalid.
 - While encoding [RenderPassEncoder].SetPipeline([Invalid RenderPipeline]).

[cwfitzgerald]

One comment, otherwise lgtm

[cwfitzgerald]

Non-blocking comment

[jinleili]

Another rare CI failure again :)

[cwfitzgerald]

Tfw rare CI failures are common - maybe if I just close my eyes and go lalala I won't consider the fact it might be a bug in wgpu - but then I would have to have even the faintest idea how to fix it....