[spv-in] Support binding arrays

[Patryk27]

This pull request extends #1845 by implementing support for binding arrays for the SPIR-V frontend 🙂

In general, the changes here boil down to adjusting parse_type_array() so that it emits crate::TypeInner::BindingArray for texture-based arrays and sampler-based array -- as you'll see in the comments, the approach implemented here is kinda hacky, because - contrary to the wgls' frontend - when parsing OpTypeArray / OpTypeRuntimeArray we can't yet know whether it's going to be used through a binding or not.

I think that ideally instead of taking a guess in parse_type_array(), we'd adjust parse_global_variable() to do something like:

if let crate::TypeInner::Array { base, size, .. } = module.types[original_ty].inner {
    if dec.desc_set.is_none() || dec.desc_index.is_none() {
        let ty2 = crate::Type {
            name: None,
            inner: crate::TypeInner::BindingArray { base, size },
        };

        ty = module.types.insert(ty2, Default::default());
    }
}

... but that doesn't work, since it leaves the original type in module.types, causing validate_type() to fail later (because it always iterates through all of the types, even if they are not actually used anywhere, as would be the case here).

I'm currently writing a raytracer using rust-gpu and I've confirmed that a shader allowed by Naga with this patch actually works (on Linux + Nvidia, I haven't checked it on Windows or Mac).

Related to:
gfx-rs/wgpu#4313.

[jimblandy]

This looks great - thank you! Just some minor changes:

[Patryk27]

Okie, ready for round 2 🙂

[Jerrody]

@teoxoy @jimblandy
I think it can be merged!

[jimblandy]

Lovely!