[metal] Retain references with command buffers

[andyleiserson]

Fixes some CTS issues, but there are still failures pending the error-reporting changes, and even after that we'd probably need to enumerate passing subcases.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.