Make SkBlitter hierarchy explicit about what needs to be implemented.

src/core/SkBlitter.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkBlitter_DEFINED
 #define SkBlitter_DEFINED
 
@@ skipping 10 matching lines @@
 struct SkMask;
 
 /** SkBlitter and its subclasses are responsible for actually writing pixels
     into memory. Besides efficiency, they handle clipping and antialiasing.
 */
 class SkBlitter {
 public:
     virtual ~SkBlitter();
 
     /// Blit a horizontal run of one or more pixels.
-    virtual void blitH(int x, int y, int width);
+    virtual void blitH(int x, int y, int width) = 0;
+
     /// Blit a horizontal run of antialiased pixels; runs[] is a *sparse*
     /// zero-terminated run-length encoding of spans of constant alpha values.
-    virtual void blitAntiH(int x, int y, const SkAlpha antialias[],
-                           const int16_t runs[]);
+    virtual void blitAntiH(int x, int y, const SkAlpha antialias[], const int16_t runs[]) = 0;
 
     /// Blit a vertical run of pixels with a constant alpha value.
     virtual void blitV(int x, int y, int height, SkAlpha alpha);
+
     /// Blit a solid rectangle one or more pixels wide.
     virtual void blitRect(int x, int y, int width, int height);
+
     /** Blit a rectangle with one alpha-blended column on the left,
         width (zero or more) opaque pixels, and one alpha-blended column
         on the right.
         The result will always be at least two pixels wide.
     */
     virtual void blitAntiRect(int x, int y, int width, int height,
                               SkAlpha leftAlpha, SkAlpha rightAlpha);
+
     /// Blit a pattern of pixels defined by a rectangle-clipped mask;
     /// typically used for text.
     virtual void blitMask(const SkMask&, const SkIRect& clip);
 
     /** If the blitter just sets a single value for each pixel, return the
         bitmap it draws into, and assign value. If not, return nullptr and ignore
         the value parameter.
     */
     virtual const SkPixmap* justAnOpaqueColor(uint32_t* value);
 
@@ skipping 175 matching lines @@
     SkBlitter*  apply(SkBlitter* blitter, const SkRegion* clip,
                       const SkIRect* bounds = nullptr);
 
 private:
     SkNullBlitter       fNullBlitter;
     SkRectClipBlitter   fRectBlitter;
     SkRgnClipBlitter    fRgnBlitter;
 };
 
 #endif