Not as straightforward as it sounds, but not hard either:
* Read OS/2 sfnt tables from TrueType fonts
* Calculate strikethrough position/thickness (prefer font-advertised if possible, calculate if not)
* Plumb the SGR code through the terminal state -- does not increase cell memory size
* Modify the shader to support it
The shaders are getting pretty nasty after this... there's tons of room for improvement. I chose to follow the existing shader style for this to keep it straightforward but will likely soon refactor the shaders.
This improves the way we calculate metrics based on fonts to be more accurate. Two practical impacts:
* We now position the underline and underline thickness based on the font metrics
* We now correctly position Apple Emoji font
And I'm going to follow this up with strikethrough since that should be possible now.
This implements font discovery so the `--font-family` flag works for macOS. Fonts are looked up using the Core Text API so any installed font on the Mac system can be used.
We still use FreeType for rendering, and CoreText doesn't _quite_ give us all the information we need to build the exact face in FreeType. So a TODO after this is to now implement glyph _rendering_ using Core Text and Core Graphics. Until then, a couple fonts don't quite work (i.e. Monaco, a big one!) but many do!
This adds the `--font-family` set of configuration flags to choose a font that is installed on the system rather than the embedded font in the binary. This uses fontconfig on Linux and only supports Linux at the moment.
Fontconfig is an optional dependency, building with `-Dfontconfig=false` will disable fontconfig and font discovery on all platforms. Fontconfig is only enabled by default on Linux.
