- Ensure the yadif .metal compiles when targeting any Metal runtime version
- Use some preprocessor awkwardness to ensure Core Video's Metal-specific
  functionality is exposed regardless of our deployment target (this works
  around what seems to be an SDK header bug, filed as FB9816002)
- Ensure all direct references to Metal functions and classes are gated
  behind runtime version checks (this satisfies clang's deployment-target
  violation warnings provided by -Wunguarded-availability).