[player] Support specifying a trace file directly

[andyleiserson]

Also provide a default initialization if the trace does not contain one.

Useful for working with fuzzing traces.

I also have some local changes for putting buffer/texture/shader data inline in the trace, but it would need some backwards compatibility logic to load existing traces before that could be merged.

Testing
Passes existing player tests, and tested playback from a file manually.

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.

[cwfitzgerald]

but it would need some backwards compatibility logic to load existing traces before that could be merged.

I don't think there's have that much interest in preserving backwards compat for traces