Add the ability to decode a subset to SkCodec.

src/codec/SkWebpCodec.cpp

Index: src/codec/SkWebpCodec.cpp
diff --git a/src/codec/SkWebpCodec.cpp b/src/codec/SkWebpCodec.cpp
index 32a8b78b14f2487e69305c6d90c89533319435bb..fea557d21ed9ddb8e9a86ff475a2c996a8fa749d 100644
--- a/src/codec/SkWebpCodec.cpp
+++ b/src/codec/SkWebpCodec.cpp
@@ -125,8 +125,26 @@ static WEBP_CSP_MODE webp_decode_mode(SkColorType ct, bool premultiply) {
 // is arbitrary.
 static const size_t BUFFER_SIZE = 4096;
 
+bool SkWebpCodec::onGetValidSubset(SkIRect* desiredSubset) const {
+    if (!desiredSubset) {
+        return false;
+    }
+
+    SkIRect bounds = SkIRect::MakeSize(this->getInfo().dimensions());
+    if (!desiredSubset->intersect(bounds)) {
+        return false;
+    }
+
+    // As stated below, libwebp snaps to even left and top. Make sure top and left are even, so we
+    // decode this exact subset.
+    // Leave right and bottom unmodified, so we suggest a slightly larger subset than requested.
+    desiredSubset->fLeft = (desiredSubset->fLeft >> 1) << 1;
+    desiredSubset->fTop = (desiredSubset->fTop >> 1) << 1;
+    return true;
+}
+
 SkCodec::Result SkWebpCodec::onGetPixels(const SkImageInfo& dstInfo, void* dst, size_t rowBytes,
-                                         const Options&, SkPMColor*, int*) {
+                                         const Options& options, SkPMColor*, int*) {
     switch (this->rewindIfNeeded()) {
         case kCouldNotRewind_RewindState:
             return kCouldNotRewind;
@@ -153,12 +171,48 @@ SkCodec::Result SkWebpCodec::onGetPixels(const SkImageInfo& dstInfo, void* dst,
     // Free any memory associated with the buffer. Must be called last, so we declare it first.
     SkAutoTCallVProc<WebPDecBuffer, WebPFreeDecBuffer> autoFree(&(config.output));
 
-    SkISize dimensions = dstInfo.dimensions();
-    if (this->getInfo().dimensions() != dimensions) {
+    SkIRect bounds = SkIRect::MakeSize(this->getInfo().dimensions());
+    if (options.fSubset) {
+        // Caller is requesting a subset.
+        if (!bounds.contains(*options.fSubset)) {
+            // The subset is out of bounds.
+            return kInvalidParameters;
+        }
+
+        bounds = *options.fSubset;
+
+        // This is tricky. libwebp snaps the top and left to even values. We could let libwebp
+        // do the snap, and return a subset which is a different one than requested. The problem
+        // with that approach is that the caller may try to stitch subsets together, and if we
+        // returned different subsets than requested, there would be artifacts at the boundaries.
+        // Instead, we report that we cannot support odd values for top and left..
+        if (!SkIsAlign2(bounds.fLeft) || !SkIsAlign2(bounds.fTop)) {
+            return kInvalidParameters;
+        }
+
+#ifdef SK_DEBUG
+        {
+            // Make a copy, since getValidSubset can change its input.
+            SkIRect subset(bounds);
+            // That said, getValidSubset should *not* change its input, in this case; otherwise
+            // getValidSubset does not match the actual subsets we can do.
+            SkASSERT(this->getValidSubset(&subset) && subset == bounds);
+        }
+#endif
+
+        config.options.use_cropping = 1;
+        config.options.crop_left = bounds.fLeft;
+        config.options.crop_top = bounds.fTop;
+        config.options.crop_width = bounds.width();
+        config.options.crop_height = bounds.height();
+    }
+
+    SkISize dstDimensions = dstInfo.dimensions();
+    if (bounds.size() != dstDimensions) {
         // Caller is requesting scaling.
         config.options.use_scaling = 1;
-        config.options.scaled_width = dimensions.width();
-        config.options.scaled_height = dimensions.height();
+        config.options.scaled_width = dstDimensions.width();
+        config.options.scaled_height = dstDimensions.height();
     }
 
     config.output.colorspace = webp_decode_mode(dstInfo.colorType(),