0.21.0 - Loops and Brick Refs

Saves now use the current game's format (brdb 1.0), so they load only on the
updated game.

• Loop Gates (default on, --driver refs): video, text-mode video and
  audio are driven by a small looping player instead of gates per pixel,
  band or speaker. A 192x108 screen is 61 gates instead of 41,487. Frames
  are stored as keyframes plus deltas (--keyframe-interval,
  --delta-threshold, --dense-keyframes). Turn it off (--driver wires)
  for the previous wired builds.
• The cost readout shows the gates run in the tick a frame changes and
  warns past ~20,000. A looped keyframe runs more gates per pixel than a
  wired one, so large looped screens can hit the server's limit.
• When the server cuts a loop short, the video's microchip shows a warning
  with how to fix it. It clears once a keyframe plays in full.
• Text mode --hold N keeps a pixel's colour until the source moves more
  than N, which makes noisy video saves much smaller.
• MIDI playheads share one function: about 4 gates a speaker instead of ~16.
  --no-midi-functions restores the old chain.
• Text components hold up to 32,767 characters (was 10,000), so text-mode
  video needs far fewer bands.
• Fixed: loop and function boards failed to paste (a gate laid out at the
  wrong size overlapped its neighbours).
• Fixed: an opened microchip board floated over the screen instead of
  sitting on its chip.
• Fixed: --prefab heightmaps lost their lowest bricks when placed on flat
  ground.
• Text-mode video's control buttons sit next to the text instead of several
  studs in front of it.
• Variable-frame-rate video gets a progress bar instead of a "total
  unknown" spinner.