Workaround wrong font height returned by freetype

The height seems to be only slightly bigger than the Ascent.
It's small enough such that characters that take up space below the
baseline (e.g. 'g', 'y', 'j', 'p') gets cut off at the bottom.
(For reference, see
https://developer.apple.com/library/archive/documentation/TextFonts/Conceptual/CocoaTextArchitecture/Art/glyph_metrics_2x.png)

This is a freetype bug: golang/freetype#34
There are multiple PRs with a fix that haven't been merged:
golang/freetype#32
golang/freetype#62

Plan9port's fontsrv also similarly sums up ascent and descent to compute
the height:
https://github.com/9fans/plan9port/blob/047fd921744f39a82a86d9370e03f7af511e6e84/src/cmd/fontsrv/x11.c#L80

font.go

@@ -65,9 +65,11 @@ func openFont(id FaceID) (*Font, error) {
 	faceCache.Lock()
 	defer faceCache.Unlock()
 	if f, ok := faceCache.m[id]; ok {
+		m := f.Metrics()
+		// TODO(fhs): Remove workaround for wrong m.Height.
 		return &Font{
 			FaceID: id,
-			Height: int(f.Metrics().Height / 64),
+			Height: (m.Ascent + m.Descent).Round(),
 			face:   f,
 		}, nil
 	}
@@ -93,9 +95,11 @@ func openFont(id FaceID) (*Font, error) {
 		face := pixFace{Face: truetype.NewFace(f, &opt)}
 		faceCache.m[id] = face
 
+		m := face.Metrics()
+		// TODO(fhs): Remove workaround for wrong m.Height.
 		return &Font{
 			FaceID: id,
-			Height: int((face.Metrics().Height + 32) / 64),
+			Height: (m.Ascent + m.Descent).Round(),
 			face:   face,
 		}, nil
 	}