Allow SPIR-V shaders to process when shader defs are present

[ProfLander]

Objective

• Prevent SPIR-V shaders from being rejected when shader defines are present.

Solution

• Have ShaderProcessor behave as a passthrough with respect to SPIR-V shaders.

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Fixes #7771

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[ProfLander]

Any chance of that review, @superdump?

If this relatively small change ends up merged, Bevy Rust-GPU will be a step closer to providing a WGSL-comparable workflow for rust-gpu shader crates in mainline bevy.

[brynnbrancamp]

This is super important for a (edit: Spirv utilizing) subset of bevy users and we need a solution.