0.5.0 — vocabulary 12: camera shake, a wider camera range, the transparent background

Two format additions; the vocabulary is unchanged and no existing reference frame moved.

• Camera shake: camera.shake = { amp, freq }, a seeded, closed-form tremble (amp up to
  0.05 of the frame, freq up to 5 per second, default 3), scaled by depth under
  parallax (#87). A camera without a shake renders exactly as before.
• Camera keyframe x/y now run from -1 to 2 instead of 0 to 1, so a camera can travel
  one and a half screens from the centre (#111). Cameras inside 0..1 are unchanged.
• Intended change: a spec stored without validation, with a camera keyframe outside
  0..1, used to be clamped to 0..1 when validated; it now renders at its stated position
  (within -1..2). Specs stored after validation already hold clamped values and do not
  change. Constitution 2.16.0.
• Transparent background: background: "transparent" with an optional ink ("light", the
  default, or "dark"). Frames carry alpha; laid over black (light) or white (dark) they equal
  the solid render, for every primitive. animspec render --png writes RGBA PNGs and --raw
  carries alpha (#100). Constitution 3.1.0.
• Cross-type reference checks now include alpha, and a pixel that is not fully opaque is
  compared by its colour weighted by alpha. Opaque pixels are compared as before, so no
  existing result changed. Constitution 3.0.0 (Principle I).
• Intended change: a spec stored without validation with background: "transparent" used
  to draw on black; it now draws transparent. Validated specs hold "black" and do not change.