Rework the font loading in the GL backend

The basic idea is to use fontdb's load_system_fonts() mechanism, in
conjunction with its built-in mmap support, to get an overview over all
installed fonts on macOS, Windows and Linux.

This isn't quite perfect in terms of discovering systems defaults, but
it's much faster than font-kit's approach of querying the system (good)
but reading the matched files into memory (not using mmap). And we have
the option of perfecting it by using fontconfig directly on Linux (where
the backend is most important).

This also paves the way for better fallback handling, as now we have a
list of all available families and we can use system APIs to query for
fallbacks.

sixtyfps_runtime/rendering_backends/gl/Cargo.toml

@@ -52,15 +52,16 @@ ttf-parser = "0.12.0"
 winit = { version = "0.25", default-features = false }
 glutin = { version = "0.27", default-features = false }
 glow = { version = "0.11.0", default-features = false }
-font-kit = { version = "0.10", features = [] }
 fontdb = { version = "0.6.2", features = ["memmap"] }
 
+[target.'cfg(target_family = "windows")'.dependencies]
+font-kit = { version = "0.10", features = [] }
+
 [target.'cfg(not(any(target_family = "windows", target_os = "macos", target_os = "ios", target_arch = "wasm32")))'.dependencies]
 # Require font-config from the system on Linux. Issue #88 indicates that the copy provided by servo-fontconfig may be incompatible
 # with distros at times.
 servo-fontconfig = { version = "0.5", features = [ "force_system_lib" ] }
-# On Windows and macOS we don't need freetype (font-kit has coretext/directwrite backends)
-font-kit = { version = "0.10", features = ["loader-freetype"] }
+libc = { version = "0.2" }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 cocoa = { version = "0.24.0" }