shiny/font: have Face.Glyph return an advance width, not a new dot.

This is consistent with Face.GlyphBounds and Face.GlyphAdvance.

Change-Id: I9da6b4f2fdb8f093fc9567c717e8fbbecc624e30
Reviewed-on: https://go-review.googlesource.com/14090
Reviewed-by: David Symonds <dsymonds@golang.org>

 font/font.go                | 11 ++++++-----
 font/plan9font/plan9font.go | 14 +++++---------
 2 files changed, 11 insertions(+), 14 deletions(-)

diff --git a/font/font.go b/font/font.go
index 8f8f4c1..a9b9729 100644
--- a/font/font.go
+++ b/font/font.go
@@ -36,8 +36,8 @@ type Face interface {
 	io.Closer
 
 	// Glyph returns the draw.DrawMask parameters (dr, mask, maskp) to draw r's
-	// glyph at the sub-pixel destination location dot. It also returns the new
-	// dot after adding the glyph's advance width.
+	// glyph at the sub-pixel destination location dot, and that glyph's
+	// advance width.
 	//
 	// It returns !ok if the face does not contain a glyph for r.
 	//
@@ -45,7 +45,7 @@ type Face interface {
 	// after the next Glyph call. Callers that want to cache the mask must make
 	// a copy.
 	Glyph(dot fixed.Point26_6, r rune) (
-		newDot fixed.Point26_6, dr image.Rectangle, mask image.Image, maskp image.Point, ok bool)
+		dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool)
 
 	// GlyphBounds returns the bounding box of r's glyph, drawn at a dot equal
 	// to the origin, and that glyph's advance width.
@@ -114,7 +114,7 @@ func (d *Drawer) DrawString(s string) {
 		if i != 0 {
 			d.Dot.X += d.Face.Kern(prevC, c)
 		}
-		newDot, dr, mask, maskp, ok := d.Face.Glyph(d.Dot, c)
+		dr, mask, maskp, advance, ok := d.Face.Glyph(d.Dot, c)
 		if !ok {
 			// TODO: is falling back on the U+FFFD glyph the responsibility of
 			// the Drawer or the Face?
@@ -122,7 +122,8 @@ func (d *Drawer) DrawString(s string) {
 			continue
 		}
 		draw.DrawMask(d.Dst, dr, d.Src, image.Point{}, mask, maskp, draw.Over)
-		d.Dot, prevC = newDot, c
+		d.Dot.X += advance
+		prevC = c
 	}
 }
 
diff --git a/font/plan9font/plan9font.go b/font/plan9font/plan9font.go
index abcc655..89f0a2d 100644
--- a/font/plan9font/plan9font.go
+++ b/font/plan9font/plan9font.go
@@ -66,19 +66,15 @@ func (f *subface) Close() error                   { return nil }
 func (f *subface) Kern(r0, r1 rune) fixed.Int26_6 { return 0 }
 
 func (f *subface) Glyph(dot fixed.Point26_6, r rune) (
-	newDot fixed.Point26_6, dr image.Rectangle, mask image.Image, maskp image.Point, ok bool) {
+	dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool) {
 
 	r -= f.firstRune
 	if r < 0 || f.n <= int(r) {
-		return fixed.Point26_6{}, image.Rectangle{}, nil, image.Point{}, false
+		return image.Rectangle{}, nil, image.Point{}, 0, false
 	}
 	i := &f.fontchars[r+0]
 	j := &f.fontchars[r+1]
 
-	newDot = fixed.Point26_6{
-		X: dot.X + fixed.Int26_6(i.width)<<6,
-		Y: dot.Y,
-	}
 	minX := int(dot.X+32)>>6 + int(i.left)
 	minY := int(dot.Y+32)>>6 + int(i.top) - f.ascent
 	dr = image.Rectangle{
@@ -91,7 +87,7 @@ func (f *subface) Glyph(dot fixed.Point26_6, r rune) (
 			Y: minY + int(i.bottom) - int(i.top),
 		},
 	}
-	return newDot, dr, f.img, image.Point{int(i.x), int(i.top)}, true
+	return dr, f.img, image.Point{int(i.x), int(i.top)}, fixed.Int26_6(i.width) << 6, true
 }
 
 func (f *subface) GlyphBounds(r rune) (bounds fixed.Rectangle26_6, advance fixed.Int26_6, ok bool) {
@@ -144,12 +140,12 @@ func (f *face) Close() error                   { return nil }
 func (f *face) Kern(r0, r1 rune) fixed.Int26_6 { return 0 }
 
 func (f *face) Glyph(dot fixed.Point26_6, r rune) (
-	newDot fixed.Point26_6, dr image.Rectangle, mask image.Image, maskp image.Point, ok bool) {
+	dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool) {
 
 	if s, rr := f.subface(r); s != nil {
 		return s.Glyph(dot, rr)
 	}
-	return fixed.Point26_6{}, image.Rectangle{}, nil, image.Point{}, false
+	return image.Rectangle{}, nil, image.Point{}, 0, false
 }
 
 func (f *face) GlyphBounds(r rune) (bounds fixed.Rectangle26_6, advance fixed.Int26_6, ok bool) {