Add subsetting to SkScaledCodec

src/codec/SkCodecPriv.h

Index: src/codec/SkCodecPriv.h
diff --git a/src/codec/SkCodecPriv.h b/src/codec/SkCodecPriv.h
index ea4aa0e092197429e183aac9bf83461c50210301..b4ccf369d4004327982e03f633bbe0d0653e7a3e 100644
--- a/src/codec/SkCodecPriv.h
+++ b/src/codec/SkCodecPriv.h
@@ -31,6 +31,18 @@
 #define COMPUTE_RESULT_ALPHA                    \
     SkSwizzler::GetResult(zeroAlpha, maxAlpha);
 
+inline bool is_valid_subset(const SkIRect* subset, const SkISize& imageDims) {
+    if (0 > subset->left() || subset->left() >= imageDims.width() ||
+            0 > subset->top() || subset->top() >= imageDims.height() ||
+            0 > subset->width() || subset->left() + subset->width() > imageDims.width() ||
+            0 > subset->height() || subset->top() + subset->height() > imageDims.height()) {
+        return false;
+    }
+    return true;
+}
+
+inline float get_scale(uint32_t sampleSize) { return 1.0f / (float) sampleSize; }
+
 /*
  * returns a scaled dimension based on the original dimension and the sampleSize
  * NOTE: we round down here for scaled dimension to match the behavior of SkImageDecoder
@@ -68,12 +80,13 @@ inline int get_dst_coord(int srcCoord, int sampleFactor) { return srcCoord / sam
  *
  * This does not need to be called and is not called when sampleFactor == 1.
  */
-inline bool is_coord_necessary(int srcCoord, int sampleFactor, int scaledDim) {
+inline bool is_coord_necessary(int srcCoord, int sampleFactor, int scaledDim, int subsetOffset) {
     // Get the first coordinate that we want to keep
-    int startCoord = get_start_coord(sampleFactor);
+    int startCoord = get_start_coord(sampleFactor) + subsetOffset;
 
     // Return false on edge cases
-    if (srcCoord < startCoord || get_dst_coord(srcCoord, sampleFactor) >= scaledDim) {
+    if (srcCoord < startCoord ||
+            get_dst_coord(srcCoord - subsetOffset, sampleFactor) >= scaledDim) {
         return false;
     }