Buffer four scanlines prior to blitting. We will eventually use this

src/core/SkBlitter.h

Index: src/core/SkBlitter.h
diff --git a/src/core/SkBlitter.h b/src/core/SkBlitter.h
index a3a2196122ee1eebe4df43d7357764c47506a9ee..0f1006fcfb70909e640f47b735771dea1b796e5a 100644
--- a/src/core/SkBlitter.h
+++ b/src/core/SkBlitter.h
@@ -33,6 +33,7 @@ public:
     /// 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[]);
+
     /// 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.
@@ -67,6 +68,14 @@ public:
     virtual bool resetShaderContext(const SkShader::ContextRec&);
     virtual SkShader::Context* getShaderContext() const;
 
+    /**
+     * Special methods for blitters that can blit more than one row at a time.
+     * This function returns the number of rows that this blitter could optimally
+     * process at a time. It is still required to support blitting one scanline
+     * at a time.
+     */
+    virtual int requestRowsPreserved() const { return 1; }
+
     ///@name non-virtual helpers
     void blitMaskRegion(const SkMask& mask, const SkRegion& clip);
     void blitRectRegion(const SkIRect& rect, const SkRegion& clip);