v0.20.0: Release 0.20.0: parallel rendering.

Parallel rendering.

Added

• render --jobs 4 renders a video in 2-second segments, several at
  once, each in its own process; the segments are joined without
  re-encoding and the soundtrack is added once. The default is one job per
  core, at most eight. A PNG sequence is split the same way.
• The file does not depend on the number of jobs: --jobs 1 and
  --jobs 8 write the same bytes. A 7-second 3D scene renders 2.7 times
  faster on 8 cores.
• oxframe_export::concat joins segments encoded with the same settings.

Changed

• A video longer than two seconds now has a key frame every two seconds
  (where the segments meet), so files are a little larger than before.