Use freetype-sys rather than Servo's freetype.

This simplifies the dependency graph as we weren't getting anything
from using the Servo `freetype` crate here as it also built and
depended upon `freetype-sys`.

Fixes #238.

harfbuzz-sys/Cargo.toml

@@ -37,12 +37,13 @@ version = "0.3"
 optional = true
 features = ["dwrite"]
 
-[dependencies.freetype]
-version = "0.7"
+[dependencies.freetype-sys]
+version = "0.18"
 optional = true
 
 [features]
 default = ["coretext", "directwrite", "freetype"]
 bundled = []
 coretext = ["core-graphics", "core-text", "foreign-types"]
 directwrite = ["winapi"]
+freetype = ["freetype-sys"]

harfbuzz-sys/src/freetype.rs

@@ -11,5 +11,5 @@ use crate::hb_font_t;
 
 extern "C" {
     /// This requires that the `freetype` feature is enabled.
-    pub fn hb_ft_font_create_referenced(face: freetype::freetype::FT_Face) -> *mut hb_font_t;
+    pub fn hb_ft_font_create_referenced(face: freetype_sys::FT_Face) -> *mut hb_font_t;
 }