Lataa agg 1.8.1

This is a quick follow up release for today's 1.8.0, which addresses a few rendering regressions:
- improved font fallback in the swash renderer: when configured font families don't contain a glyph, agg now tries other loaded system and --font-dir fonts before giving up
- improved font fallback in the resvg renderer to prefer agg's configured fallback chain before falling back to arbitrary fonts from the font database
- fixed clipping/overpainting of glyphs that extend beyond their own terminal cell in the swash renderer
The highlights from 1.8.0:
- added new swash font/text rendering backend, now the default, replacing fontdue; --renderer fontdue remains accepted as a compatibility alias
- improved font fallback, with built-in Noto Emoji and Nerd Font Symbols fallback fonts
- added --text-font-family and --emoji-font-family options for finer control over font selection; --font-family still works as an all-in-one override
- improved rendering of emoji, Nerd Font icons, CJK characters, bold/italic text, underlines, and block/mosaic drawing characters
- added explicit support for Apple Color Emoji on macOS
- added fallback handling for unsupported COLRv1 color emoji glyphs
- added --bold-is-bright option to render bold text using bright ANSI colors
- fixed faint text attribute and underlined spaces rendering in the resvg renderer
- --line-height values below 1.0 are now rejected with a clear error message
- upgraded avt to latest version to bring recent terminal emulation improvements
- upgraded rendering, CLI, HTTP, logging, and Nix-related dependencies
- updated Nix package/development setup