v0.21.1

Changed

• The logo is transparent. Both copies of the mark — assets/vgi-logo.png,
  which the crate READMEs link, and the data: URI inlined in the landing page
  vgi-rpc serves — were the old export on a white background with no alpha, so
  both wore a white rectangle wherever the page behind them was not white. Both
  are now cut from a committed master by scripts/regenerate_logo_assets.py.
  The inlined copy is palettized to 256 colours, which for flat artwork is
  visually indistinguishable from truecolour and holds the base64 compiled into
  every dependent binary at 20 KiB rather than 99 KiB.

No API or wire change; this is a patch.