Improve freetype metric usage

The font metrics function was using freetype metrics in an ineffective
way, improve the use of those metrics and remove the now unnecessary
separate default values for font offset in linux.

alacritty.yml

@@ -57,8 +57,8 @@ font:
   # Offset is the extra space around each character. offset.y can be thought of
   # as modifying the linespacing, and offset.x as modifying the letter spacing.
   offset:
-    x: 2.0
-    y: -7.0
+    x: 0.0
+    y: 0.0
 
   # OS X only: use thin stroke font rendering. Thin strokes are suitable
   # for retina displays, but for non-retina you probably want this set to

font/src/ft/mod.rs

@@ -59,18 +59,15 @@ impl ::Rasterize for FreeTypeRasterizer {
             .get(&key)
             .ok_or(Error::FontNotLoaded)?;
 
-        let scale_size = self.dpr as f64 * size.as_f32_pts() as f64;
+        let size_metrics = face.size_metrics()
+            .ok_or(Error::MissingSizeMetrics)?;
 
-        let em_size = face.em_size() as f64;
-        let w = face.max_advance_width() as f64;
-        let h = (face.ascender() - face.descender() + face.height()) as f64;
-
-        let w_scale = w * scale_size / em_size;
-        let h_scale = h * scale_size / em_size;
+        let width = (size_metrics.max_advance / 64) as f64;
+        let height = (size_metrics.height / 64) as f64;
 
         Ok(Metrics {
-            average_advance: w_scale,
-            line_height: h_scale,
+            average_advance: width,
+            line_height: height,
         })
     }
 
@@ -278,6 +275,9 @@ pub enum Error {
     /// Couldn't find font matching description
     MissingFont(FontDesc),
 
+    /// Tried to get size metrics from a Face that didn't have a size
+    MissingSizeMetrics,
+
     /// Requested an operation with a FontKey that isn't known to the rasterizer
     FontNotLoaded,
 }
@@ -295,6 +295,7 @@ impl ::std::error::Error for Error {
             Error::FreeType(ref err) => err.description(),
             Error::MissingFont(ref _desc) => "couldn't find the requested font",
             Error::FontNotLoaded => "tried to operate on font that hasn't been loaded",
+            Error::MissingSizeMetrics => "tried to get size metrics from a face without a size",
         }
     }
 }
@@ -311,6 +312,9 @@ impl ::std::fmt::Display for Error {
             },
             Error::FontNotLoaded => {
                 f.write_str("Tried to use a font that hasn't been loaded")
+            },
+            Error::MissingSizeMetrics => {
+                f.write_str("Tried to get size metrics from a face without a size")
             }
         }
     }

src/config.rs

@@ -1189,6 +1189,12 @@ impl FontOffset {
     }
 }
 
+impl Default for FontOffset {
+    fn default() -> FontOffset {
+        FontOffset { x: 0.0, y: 0.0 }
+    }
+}
+
 trait DeserializeFromF32 : Sized {
     fn deserialize_from_f32<D>(D) -> ::std::result::Result<Self, D::Error>
         where D: serde::de::Deserializer;
@@ -1301,10 +1307,7 @@ impl Default for Font {
             italic: FontDescription::new_with_family("Menlo"),
             size: Size::new(11.0),
             use_thin_strokes: true,
-            offset: FontOffset {
-                x: 0.0,
-                y: 0.0
-            }
+            offset: Default::default()
         }
     }
 }
@@ -1318,12 +1321,7 @@ impl Default for Font {
             italic: FontDescription::new_with_family("monospace"),
             size: Size::new(11.0),
             use_thin_strokes: false,
-            offset: FontOffset {
-                // TODO should improve freetype metrics... shouldn't need such
-                // drastic offsets for the default!
-                x: 2.0,
-                y: -7.0
-            }
+            offset: Default::default()
         }
     }
 }