freetype/truetype: adjust ends slice when hinting a compound glyph.

Successfully hint all of DejaVu Sans Oblique, barring 4 exceptions
that are errors in the upstream C Freetype project. See
http://lists.nongnu.org/archive/html/freetype/2013-11/msg00004.html
for details.

R=bsiegert
CC=golang-dev, remyoudompheng
https://codereview.appspot.com/21580044

freetype/truetype/glyph.go

@@ -358,15 +358,29 @@ func (g *GlyphBuf) loadCompound(recursion int32, b Bounds, uhm HMetric, i Index,
 	}
 	program := glyf[offset : offset+instrLen]
 	g.addPhantomsAndScale(b, uhm, i, len(g.Point), false)
-	points := g.Point[np0:]
+	points, ends := g.Point[np0:], g.End[ne0:]
 	g.Point = g.Point[:len(g.Point)-4]
 	for j := range points {
 		points[j].Flags &^= flagTouchedX | flagTouchedY
 	}
+	// Temporarily adjust the ends to be relative to this compound glyph.
+	if np0 != 0 {
+		for i := range ends {
+			ends[i] -= np0
+		}
+	}
 	// Hinting instructions of a composite glyph completely refer to the
 	// (already) hinted subglyphs.
 	g.tmp = append(g.tmp[:0], points...)
-	return g.hinter.run(program, points, g.tmp, g.tmp, g.End[ne0:])
+	if err := g.hinter.run(program, points, g.tmp, g.tmp, ends); err != nil {
+		return err
+	}
+	if np0 != 0 {
+		for i := range ends {
+			ends[i] += np0
+		}
+	}
+	return nil
 }
 
 func (g *GlyphBuf) addPhantomsAndScale(b Bounds, uhm HMetric, i Index, np0 int, appendOther bool) {

freetype/truetype/truetype_test.go

@@ -256,11 +256,22 @@ var scalingTestCases = []struct {
 }{
 	{"luxisr", 12, -1},
 	{"x-arial-bold", 11, 0},
-	{"x-deja-vu-sans-oblique", 17, 2077},
+	{"x-deja-vu-sans-oblique", 17, -1},
 	{"x-droid-sans-japanese", 9, 0},
 	{"x-times-new-roman", 13, 0},
 }
 
+var scalingExceptions = map[string]map[int]bool{
+	// TODO: remove these exceptions when C Freetype version 2.5.1 is released:
+	// see http://lists.nongnu.org/archive/html/freetype/2013-11/msg00004.html
+	"x-deja-vu-sans-oblique": map[int]bool{
+		2077: true,
+		2078: true,
+		2171: true,
+		2172: true,
+	},
+}
+
 // TODO: also test bounding boxes, not just points.
 
 func testScaling(t *testing.T, hinter *Hinter) {
@@ -296,6 +307,7 @@ func testScaling(t *testing.T, hinter *Hinter) {
 			continue
 		}
 
+		exceptions := scalingExceptions[tc.name]
 		glyphBuf := NewGlyphBuf()
 		for i, want := range wants {
 			// TODO: completely implement hinting. For now, only the first
@@ -304,6 +316,10 @@ func testScaling(t *testing.T, hinter *Hinter) {
 				break
 			}
 
+			if exceptions != nil && exceptions[i] {
+				continue
+			}
+
 			if err = glyphBuf.Load(font, tc.size*64, Index(i), hinter); err != nil {
 				t.Errorf("%s: glyph #%d: Load: %v", tc.name, i, err)
 				continue