font: add Metrics.Descent.

Change-Id: Id38e7e54264e0e77a28afd8e10cf574610b47336
Reviewed-on: https://go-review.googlesource.com/21019
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

font/basicfont/basicfont.go

@@ -75,6 +75,7 @@ func (f *Face) Metrics() font.Metrics {
 	return font.Metrics{
 		Height:  fixed.I(f.Height),
 		Ascent:  fixed.I(f.Ascent),
+		Descent: fixed.I(f.Height - f.Ascent),
 	}
 }
 

font/font.go

@@ -74,11 +74,17 @@ type Face interface {
 // Metrics holds the metrics for a Face. A visual depiction is at
 // https://developer.apple.com/library/mac/documentation/TextFonts/Conceptual/CocoaTextArchitecture/Art/glyph_metrics_2x.png
 type Metrics struct {
+	// Height is the recommended amount of vertical space between two lines of
+	// text.
+	Height fixed.Int26_6
+
 	// Ascent is the distance from the top of a line to its baseline.
 	Ascent fixed.Int26_6
 
-	// Height is the recommended amount of vertical space between two lines of text.
-	Height fixed.Int26_6
+	// Descent is the distance from the bottom of a line to its baseline. The
+	// value is typically positive, even though a descender goes below the
+	// baseline.
+	Descent fixed.Int26_6
 }
 
 // TODO: Drawer.Layout or Drawer.Measure methods to measure text without

font/plan9font/plan9font.go

@@ -69,6 +69,7 @@ func (f *subface) Metrics() font.Metrics {
 	return font.Metrics{
 		Height:  fixed.I(f.height),
 		Ascent:  fixed.I(f.ascent),
+		Descent: fixed.I(f.height - f.ascent),
 	}
 }
 
@@ -150,6 +151,7 @@ func (f *face) Metrics() font.Metrics {
 	return font.Metrics{
 		Height:  fixed.I(f.height),
 		Ascent:  fixed.I(f.ascent),
+		Descent: fixed.I(f.height - f.ascent),
 	}
 }