Fix hlsl storage format generation

[Vecvec]

Connections

Description
Previously we generated a vec2 for rg8unorm on hlsl as the storage type; however, this is for the output variable, not the stored type. This PR makes this always a vec4 unless it's a one length type because: 1. They seem to be auto convertible to a vec4 2. it breaks image atomics. I would prefer these to be vec4s too but I can't find a way to not break image atomics.

Testing
Adds a new naga snapshot which loads and stores to storage textures which are r8unorm, rg8unorm and rgba8unorm.

Checklist

• Run cargo fmt.
• [n/a] Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[cwfitzgerald]

Have you tested this on an example that is actually run?