message: Changelog.md: Adds notes on PTY buffer objects and its use.
diff: - Adds new configuration option `renderer.tile_cache_count` to configure texture atlas cache capacity.
  - Adds new configuration option `renderer.tile_direct_mapping` to enable/disalbe texture atlas direct mapping.
  - Adds new colorscheme setting `background_image.path` and `background_image.opacity` to optionally render a terminal background image.
  +- Adds stdout-fastpipe support on new shell file descriptor `3` and exposing availability via
  +- Adds new configuration option `pty_buffer_size` for tweaking the new size in bytes for the PTY buffer objects.
  +environment variable `STDOUT_FASTPIPE` (with value `3`). When writing to this file descriptor rather
  +than to stdout (file descriptor `1`), the full performance of Contour can be explored.
  - Adds `mock` font locator (mostly interesting for advanced use for CI testing).
  - Adds VT sequence `SM ? 8452 h` / `RM ? 8452 l` for enabling/disabling sixel cursor placement conformance (xterm extension).
  - Adds SGR-Pixels support, VT sequence `SM ? 1016 h` / `RM ? 1016 l` (#574).
  