Allow sampling GIF images during decode.

src/images/SkScaledBitmapSampler.h

Index: src/images/SkScaledBitmapSampler.h
diff --git a/src/images/SkScaledBitmapSampler.h b/src/images/SkScaledBitmapSampler.h
index a293fe7ebb728d2ea88d1f95fbc3f5f341231042..e6c4577174c3551afbdf1b228b658d1b668113fe 100644
--- a/src/images/SkScaledBitmapSampler.h
+++ b/src/images/SkScaledBitmapSampler.h
@@ -22,6 +22,7 @@ public:
     int scaledHeight() const { return fScaledHeight; }
 
     int srcY0() const { return fY0; }
+    int srcDX() const { return fDX; }
     int srcDY() const { return fDY; }
 
     enum SrcConfig {
@@ -42,6 +43,12 @@ public:
     // returns true if the row had non-opaque alpha in it
     bool next(const uint8_t* SK_RESTRICT src);
 
+    // Like next(), but specifies the y value of the source row, so the
+    // rows can come in any order. If the row is not part of the output
+    // sample, it will be skipped. Only sampleInterlaced OR next should
+    // be called for one SkScaledBitmapSampler.
+    bool sampleInterlaced(const uint8_t* SK_RESTRICT src, int srcY);
+
     typedef bool (*RowProc)(void* SK_RESTRICT dstRow,
                             const uint8_t* SK_RESTRICT src,
                             int width, int deltaSrc, int y,
@@ -56,6 +63,18 @@ private:
     int fDX;    // step between X samples
     int fDY;    // step between Y samples
 
+#ifdef SK_DEBUG
+    // Keep track of whether the caller is using next or sampleInterlaced.
+    // Only one can be used per sampler.
+    enum SampleMode {
+        kUninitialized_SampleMode,
+        kConsecutive_SampleMode,
+        kInterlaced_SampleMode,
+    };
+
+    SampleMode fSampleMode;
+#endif
+
     // setup state
     char*   fDstRow; // points into bitmap's pixels
     size_t  fDstRowBytes;