Populate sampling pairs for validation

[kvark]

Connections
Fixes #2182

Description
We didn't populate the set

Testing
Modified the skybox example:

Caused by:
    In Device::create_render_pipeline
      note: label = `Sky`
    error matching FRAGMENT shader requirements against the pipeline
    unable to filter the texture (ResourceBinding { group: 0, binding: 1 }) by the sampler (ResourceBinding { group: 0, binding: 2 })
    non-filterable float texture

[Jerrody]

Hey, in Rust was added some syntax sugar with that you can iter over an array without explicitly writing iter().
Like:

            for key in info.sampling_set {
                ep.sampling_pairs
                    .insert((resource_mapping[&key.image], resource_mapping[&key.sampler]));
            }

[kvark]

Oh yes, I'm well aware, just like my iterators explicit.

[kvark]

Published in wgpu-core-0.11.3