dm: simplify parallel/serial decisions

  - instead of complicated enclaves, just allow parallel or main thread serial
  - either the Src or the Sink can force the task to go serial

ALSO,
  - simpler file extension parsing

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1675423002

Review URL: https://codereview.chromium.org/1675423002
3 files changed