Add scanline decoding to SkCodec.

src/codec/SkSwizzler.h

Index: src/codec/SkSwizzler.h
diff --git a/src/codec/SkSwizzler.h b/src/codec/SkSwizzler.h
index 0bf2ee306a9377c64e5691260b2e9272ea853e06..cd5ba4757c4a852d151caf9c51e8128b38c17daa 100644
--- a/src/codec/SkSwizzler.h
+++ b/src/codec/SkSwizzler.h
@@ -18,6 +18,7 @@ public:
      *  Enum describing the config of the source data.
      */
     enum SrcConfig {
+        kUnknown,  // Invalid type.
         kGray,  // 1 byte per pixel
         kIndex, // 1 byte per pixel
         kRGB,   // 3 bytes per pixel
@@ -64,6 +65,17 @@ public:
      *  @return Whether the row had non-opaque alpha.
      */
     bool next(const uint8_t* SK_RESTRICT src);
+
+    /**
+     *  Update the destination row.
+     *
+     *  Typically this is done by next, but for a client that wants to manually
+     *  modify the destination row (for example, for decoding scanline one at a
+     *  time) they can call this before each call to next.
+     *  TODO: Maybe replace this with a version of next which allows supplying the
+     *  destination?
+     */
+    void setDstRow(void* dst) { fDstRow = dst; }
 private:
     /**
      *  Method for converting raw data to Skia pixels.