add helper to create fancy underlines

src/core/SkGlyph.h

Index: src/core/SkGlyph.h
diff --git a/src/core/SkGlyph.h b/src/core/SkGlyph.h
index 92f974bc2cf456f8ed367c5f8d0c676baa8a2f26..280e5bc5f1d918a4c8f01e4ff687817e647967f9 100644
--- a/src/core/SkGlyph.h
+++ b/src/core/SkGlyph.h
@@ -34,12 +34,24 @@ class SkGlyph {
         kSubShiftY = 0
     };
 
- public:
+public:
+    // Support horizontal and vertical skipping strike-through / underlines.
+    // The caller walks the linked list looking for a match. For a horizontal underline,
+    // the fBounds contains the top and bottom of the underline. The fInterval pair contains the
+    // beginning and end of of the intersection of the bounds and the glyph's path.
+    // If interval[0] >= interval[1], no intesection was found.
+    struct Intercept {
+        Intercept* fNext;
+        SkScalar fBounds[2];    // for horz underlines, the boundaries in Y
+        SkScalar fInterval[2];  // the outside intersections of the axis and the glyph
+    };
+
     static const SkFixed kSubpixelRound = SK_FixedHalf >> SkGlyph::kSubBits;
     // A value that can never be generated by MakeID.
     static const uint32_t kImpossibleID = ~0;
     void*       fImage;
     SkPath*     fPath;
+    Intercept*  fIntercept;

[reed1, 2016/02/05 18:56:59]

Hmmm, we already have too much stored in Glyphs, since we typically never have
both a path and an image-ptr. This adds another ptr that will almost always be
null. I wonder if we can find a different way to cache this information (e.g.
separate key, so that the value need not be SkGlyph per-se.)

[caryclark, 2016/02/05 19:43:36]

I can imagine something that looks like

struct GlyphData {
   Type fType;
   union {
      void* fImage;
      SkPath* fPath;
      Intercept* fIntercept;
   };
};

and either the struct or a pointer to it resides in the struct.

Would you like that remedied in this CL or a separate CL?

     SkFixed     fAdvanceX, fAdvanceY;
 
     uint16_t    fWidth, fHeight;
@@ -133,6 +145,7 @@ class SkGlyph {
     void initCommon(uint32_t id) {
         fID             = id;
         fImage          = nullptr;
+        fIntercept      = nullptr;
         fPath           = nullptr;
         fMaskFormat     = MASK_FORMAT_UNKNOWN;
         fForceBW        = 0;