Urgent
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- Add a --diff option to crucible-run that dumps image diffs for failed tests.
  (Requires multiple command buffers).
- Add an option to crucible-run that selects how the runner isolates tests:
  one thread per test, one process per test, or no isolation.

Rainy Day
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- Add an example multi-threaded test, "example.threads".
- Consolidate some crucible headers. There are too many.
- Clean up the messy cru_image code.