Add subsetting to SkScaledCodec

tools/SkBitmapRegionDecoderPriv.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkBitmapRegionDecoderPriv_DEFINED
+#define SkBitmapRegionDecoderPriv_DEFINED
+
+/*
+ * Chooses the correct image subset offsets and dimensions for the partial decode.
+ *
+ * @return true if the subset is completely contained within the image
+ *         false otherwise
+ */
+inline bool set_subset_region(int inputOffset, int inputDimension,
+        int imageOriginalDimension, int* imageSubsetOffset, int* outOffset,
+        int* imageSubsetDimension) {
+
+    // This must be at least zero, we can't start decoding the image at a negative coordinate.
+    *imageSubsetOffset = SkTMax(0, inputOffset);
+
+    // If inputOffset is less than zero, we decode to an offset location in the output bitmap.
+    *outOffset = *imageSubsetOffset - inputOffset;
+
+    // Use imageSusetOffset to make sure we don't decode pixels past the edge of the image.
+    // Use outOffset to make sure we don't decode pixels past the edge of the region.
+    *imageSubsetDimension = SkTMin(imageOriginalDimension - *imageSubsetOffset,
+            inputDimension - *outOffset);
+
+    return (*outOffset == 0) && (*imageSubsetDimension == inputDimension);
+}
+
+#endif // SkBitmapRegionDecoderPriv_DEFINED