v0.6.0

What an application drew, as against how many bytes it spent drawing it.

Inline graphics were observable only as a count and a size: an image had
gone out, and it had been about so big. Three things were wrong with that,
and the first two were wrong rather than merely thin — the count was of
escapes, not of images, so the kitty protocol's own 4096-byte chunking
inflated it and a delete posed as a transmission.

Added

• GraphicsSeen::payloads — the transmissions themselves. Each
  GraphicsPayload carries its protocol, action, format, compression,
  image id, the pixel size and cell extent the application declared, the
  bytes it cost, the chunks it took, and the data itself. Placement is the
  one fact that lives in the grid rather than in the payload, so at()
  reports the cursor position at the terminator — the image's top-left
  corner for both protocols. An application that lays out in characters and
  draws in pixels can now be held to keeping the two in step, which is a
  failure nothing on screen shows: a picture that slides out from under its
  own labels leaves every cell exactly as it was.
• GraphicsSeen::deletes, counting kitty a=d — images taken off
  the screen — apart from images transmitted.
• The decode feature: GraphicsPayload::decode and Bitmap. Kitty
  f=24/f=32, zlib'd or not, and the sixel data stream decode into
  pixels, so an assertion can be about the picture rather than about its
  size. Off by default: it is the one thing here needing a dependency of
  its own (zlib), and every other fact about a payload stays free. Refusals
  name their reason — f=100 (PNG) is unsupported rather than guessed, a
  delete carries no image, and a payload past the capture bound says so
  instead of decoding a prefix of itself into a plausible wrong picture.
• TerminalBuilder::capture_graphics, the retention budget: 4 MiB by
  default, 0 to keep counts and drop every byte. Bounded like scrollback,
  and the counters stay exact whatever the bound.
• The image-echo fixture, which transmits a known image over kitty
  (compressed, plain, and chunked), over sixel, and with a delete after it.

Fixed

• A chunked kitty transmission is one image, not one per escape. The
  protocol caps a payload at 4096 bytes and continues with m=1, so a
  4.9 KB chart counted as two images and the continuations — which carry no
  control block — counted as pictures nothing could be said about. The
  chunks are joined before anything is counted.
• A kitty delete is no longer counted as an image transmitted. a=d
  carries no picture. Every byte of it is still counted in bytes(): a
  delete is traffic.

Changed

• GraphicsSeen is Clone rather than Copy, since it now carries the
  payload list. Existing code that reads a counter is unaffected; code that
  copied the value into two bindings needs a clone.