Requires Rust-GPU/rust-gpu#496 and rev update

• cargo gpu clippy allows you to run clippy with a spirv target, which can check code hidden by #[cfg(target_arch = "spirv")], that cargo clippy --all-targets does not.

• Also adds cargo gpu check, not sure how useful it'll be.

• WON'T add cargo gpu fmt since cargo fmt is fully target independent. We could use the fmt of the rust-gpu rust toolchain, but at worst that'll get you different formatting from whatever stable toolchain you're using.

• enabled by

  • custom cargo commands in No longer require crate-type = ["dylib"] on shader crates rust-gpu#477
  • proper no artifact handing in spirv-builder: when no artifact is produced, error instead of panic rust-gpu#496