Specify that filtering with comparison is implementation-defined

[kainino0x]

And specify that comparison+filtering is always allowed, but non-comparison+filtering is never allowed. Investigation indicates this is the best we can support on Vulkan for any format.

Fixes #1266

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[kvark]

if we allow depth textures bound as regular float textures, we could still support linear sampling, couldn't we?
The backend, seeing a depth texture, may create a view as non-depth. And that R-only view is probably filterable.

[kainino0x]

if we allow depth textures bound as regular float textures, we could still support linear sampling, couldn't we? The backend, seeing a depth texture, may create a view as non-depth. And that R-only view is probably filterable.

Hm, I didn't think so, but maybe I'm missing something. According to #744 (comment), Vulkan doesn't allow reinterpreting depth formats. Also, I don't think we would know which view to use at createBindGroup time unless we can always use an R-only view?

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[kainino0x]

I realized I needed to actually update the associated algorithm to disallow filtering "depth" bindings. (#2094 to determine whether depth textures themselves can be used with "unfilterable-float" bindings.)

[kainino0x]

oops, apparently the review from kvark was still valid for automerge. @toji PTAL anyway

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[toji]

Post-merge LGTM. :)