textRenderingMsdf is broken

[greggman]

https://webgpu.github.io/webgpu-samples/?sample=textRenderingMsdf gets the following error

GPUPipelineError: fine derivative builtins (dpdxFine, dpdyFine, fwidthFine) can not be used in compatibility mode in stage (Fragment), entry point "fragmentMain"

What's the proper fix?

1. make it require core
2. use dpdx, dpdy, and fwidth instead of the fine versions
3. choose fine versions on core and non-fine on compat
4. ???

[kainino0x]

• use dpdx, dpdy, and fwidth instead of the fine versions

I would do this one as long as coarse derivatives work OK for the sample. We can test by switching to the Coarse versions and running it on some devices that has actually-coarse derivatives. Offhand, the only place I know this is supposed to be true is D3D, see gpuweb/gpuweb#4325.

[toji]

For what it's worth the original sample didn't include the derivatives at all, they were introduced in #405 by @rconde01 to improve the aliasing of the text. Their implementation references Chlumsky/msdfgen#22 (comment) as the source of the AA algorithm. It did make a very noticeable difference in text clarity, so we want to keep it as much as possible.

It's not clear to me from reading that thread how impactful using the coarse derivatives would be, so we'll just have to give it a try as Kai mentioned.

[toji]

This should be resolved now with Gregg's fix in #556