v1.7.1

Fixes video redaction failing on macOS with GPU acceleration, and stops a
redaction mask from sliding into place when a new face appears.

Fixes

• Video redaction on macOS no longer stops with a CoreML error partway
  through: the face and license-plate models that Apple's GPU backend can't
  run now fall back to the CPU automatically, while the models it can run
  stay on the GPU
• A newly appeared face is now covered where it appears, instead of the mask
  sliding in from where a different face left the frame

Details

• Each detector is exercised once as it loads; if the GPU backend rejects it,
  that detector is rebuilt on the CPU. Photo detection, the fast 640 px face
  model, and the Windows/Linux GPU paths are unaffected
• Detection that falls back to the CPU now runs across all cores instead of a
  single thread, cutting the accurate face model's analysis time on
  high-resolution video by roughly two-thirds
• A track that loses its face no longer drifts across the frame to grab an
  unrelated new one, and a gap that would imply an implausibly fast jump is
  left uninterpolated rather than drawn as a slide