Validate storage texture formats when creating BGLs

[andyleiserson]

We were validating storage texture compatibility when creating bindings, but not when creating BGLs. The spec requires both. Extracts the existing validation for bindings into a helper function and calls it when creating a BGL.

I omitted a changelog entry since it seems unlikely that anybody could be accomplishing anything useful by creating such BGLs and not binding them, but, I'd be happy to add one if it seems appropriate. (Not specific to this change, but in general, stricter validation seems likely to break things and thus worth noting in the change log. On the other hand, we have a lot of missing validation so noting absolutely every change to validation could bloat the changelog quite a bit.)

Testing
By the CTS.

Squash or Rebase? Squash

Checklist

• If this contains user-facing changes, add a CHANGELOG.md entry.

[andyleiserson]

I think it probably makes sense to move this helper to a free function in conv rather than being an associated method on Device. The reason I haven't is that I briefly considered whether the call to describe_format_features should be inside the helper, in which case a method on Device would be needed. So I thought I would wait and see if there is review feedback.

[andyleiserson]

I added this here to avoid match-conversion of errors returned by the helper or introducing a new error type. But I can see a case that it makes more sense to preserve BindingError exclusively for bindings and not binding templates.

[teoxoy]

The spec doesn't seem to require validating this twice. The format is validated when creating the BGL and when creating the BG, the format of the view is checked against the format of the BGL entry.