Render underline/strikethrough as rects

Support for strikethrough has been added by inserting and removing a
`STRIKE_THROUGH` flag on the cell.

Now all strikethrough and underline drawing is also done through the
rectangle renderer. So no glyphs are used to render underlines and
strikethrough.
The position is taken from the font metrics and should be accurate for
linux, however is not yet tested on macos.

It works by checking the underline state for each cell and then drawing
from the start until the last position whenever an underline ended. This
adds a few checks even if no underline is rendered but I was not able to
measure any significant performance impact.

Fixes jwilm/alacritty#806.
Fixes jwilm/alacritty#31.

3 files changed, 15 insertions, 0 deletions

diff --git a/font/src/darwin/mod.rs b/font/src/darwin/mod.rs
index 14381113..89858f77 100644
--- a/font/src/darwin/mod.rs
+++ b/font/src/darwin/mod.rs
@@ -422,11 +422,15 @@ impl Font {
         let descent = self.ct_font.descent() as f64;
         let leading = self.ct_font.leading() as f64;
         let line_height = (ascent + descent + leading + 0.5).floor();
+        let underline_position = self.ct_font.underline_position() as f32;
+        let underline_thickness = self.ct_font.underline_thickness() as f32;
 
         Metrics {
             average_advance: average_advance,
             line_height: line_height,
             descent: -(self.ct_font.descent() as f32),
+            underline_position,
+            underline_thickness,
         }
     }
 
diff --git a/font/src/ft/mod.rs b/font/src/ft/mod.rs
index 68d2faf3..6d77a61c 100644
--- a/font/src/ft/mod.rs
+++ b/font/src/ft/mod.rs
@@ -86,15 +86,24 @@ impl ::Rasterize for FreeTypeRasterizer {
     }
 
     fn metrics(&self, key: FontKey) -> Result<Metrics, Error> {
+        let face = self.faces
+            .get(&key)
+            .ok_or(Error::FontNotLoaded)?;
         let full = self.full_metrics(key)?;
 
         let height = (full.size_metrics.height / 64) as f64;
         let descent = (full.size_metrics.descender / 64) as f32;
+        let underline_position =
+            (f32::from(face.ft_face.underline_position()) / 64.).round();
+        let underline_thickness =
+            (f32::from(face.ft_face.underline_thickness()) / 64.).round();
 
         Ok(Metrics {
             average_advance: full.cell_width,
             line_height: height,
             descent: descent,
+            underline_position,
+            underline_thickness,
         })
     }
 
diff --git a/font/src/lib.rs b/font/src/lib.rs
index 330ed362..7a8ac7a7 100644
--- a/font/src/lib.rs
+++ b/font/src/lib.rs
@@ -314,6 +314,8 @@ pub struct Metrics {
     pub average_advance: f64,
     pub line_height: f64,
     pub descent: f32,
+    pub underline_position: f32,
+    pub underline_thickness: f32,
 }
 
 pub trait Rasterize {