* System font API
This is a new API that allows operating system fonts to be loaded by the engine and used by content.
Fonts are provided in a flat list exposing all the relevant metadata. They are loaded from disk with a Load call.
Initial implementation is only for Windows DirectWrite.
* Load system fonts as memory mapped files if possible.
This allows sharing the font file memory with other processes which is always good.
* Use ArrayPool to reduce char array allocations
* Disable verbose logging
* Implement system font support on Linux via Fontconfig
* Implement macOS support
* Add "FREEDESKTOP" define constant
This is basically LINUX || FREEBSD. Though FreeBSD currently gets detected as LINUX too. Oh well.
* Compile out Fontconfig and CoreText system font backends when not on those platforms
* Don't add Fontconfig package dep on Mac/Windows
* Allow disabling system font support via CVar
Cuz why not.
