text: Add FaceWithLineHeight

Closes #1760

examples/text/main.go

@@ -108,6 +108,13 @@ func (g *Game) Draw(screen *ebiten.Image) {
 		op.Filter = ebiten.FilterLinear
 		text.DrawWithOptions(screen, sampleText, mplusNormalFont, op)
 	}
+	{
+		const x, y = 160, 240
+		const lineHeight = 80
+		b := text.BoundString(text.FaceWithLineHeight(mplusBigFont, lineHeight), sampleText)
+		ebitenutil.DrawRect(screen, float64(b.Min.X+x), float64(b.Min.Y+y), float64(b.Dx()), float64(b.Dy()), gray)
+		text.Draw(screen, sampleText, text.FaceWithLineHeight(mplusBigFont, lineHeight), x, y, color.White)
+	}
 }
 
 func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {

text/text.go

@@ -352,3 +352,43 @@ func CacheGlyphs(face font.Face, text string) {
 		getGlyphImage(face, r)
 	}
 }
+
+// FaceWithLineHeight returns a font.Face with the given lineHeight in pixels.
+// The returned face will otherwise have the same glyphs and metrics as face.
+func FaceWithLineHeight(face font.Face, lineHeight float64) font.Face {
+	return faceWithLineHeight{
+		face:       face,
+		lineHeight: fixed.Int26_6(lineHeight * (1 << 6)),
+	}
+}
+
+type faceWithLineHeight struct {
+	face       font.Face
+	lineHeight fixed.Int26_6
+}
+
+func (f faceWithLineHeight) Close() error {
+	return f.face.Close()
+}
+
+func (f faceWithLineHeight) Glyph(dot fixed.Point26_6, r rune) (dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool) {
+	return f.face.Glyph(dot, r)
+}
+
+func (f faceWithLineHeight) GlyphBounds(r rune) (bounds fixed.Rectangle26_6, advance fixed.Int26_6, ok bool) {
+	return f.face.GlyphBounds(r)
+}
+
+func (f faceWithLineHeight) GlyphAdvance(r rune) (advance fixed.Int26_6, ok bool) {
+	return f.face.GlyphAdvance(r)
+}
+
+func (f faceWithLineHeight) Kern(r0, r1 rune) fixed.Int26_6 {
+	return f.face.Kern(r0, r1)
+}
+
+func (f faceWithLineHeight) Metrics() font.Metrics {
+	m := f.face.Metrics()
+	m.Height = f.lineHeight
+	return m
+}