Add the ability to decode a subset to SkCodec.

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index cc635e012ca82bd2c11839ab10fe51ab5f9625a7..ab2033948fa476ce0eea146d62530aafaf954acb 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -58,6 +58,25 @@ public:
     }
 
     /**
+     *  Return (via desiredSubset) a subset which can decoded from this codec,
+     *  or false if this codec cannot decode subsets or anything similar to
+     *  desiredSubset.
+     *
+     *  @param desiredSubset In/out parameter. As input, a desired subset of
+     *      the original bounds (as specified by getInfo). If true is returned,
+     *      desiredSubset may have been modified to a subset which is
+     *      supported. Although a particular change may have been made to
+     *      desiredSubset to create something supported, it is possible other
+     *      changes could result in a valid subset.
+     *      If false is returned, desiredSubset's value is undefined.
+     *  @return true if this codec supports decoding desiredSubset (as
+     *      returned, potentially modified)
+     */
+    bool getValidSubset(SkIRect* desiredSubset) const {
+        return this->onGetValidSubset(desiredSubset);
+    }
+
+    /**
      *  Format of the encoded data.
      */
     SkEncodedFormat getEncodedFormat() const { return this->onGetEncodedFormat(); }
@@ -128,9 +147,21 @@ public:
      */
     struct Options {
         Options()
-            : fZeroInitialized(kNo_ZeroInitialized) {}
+            : fZeroInitialized(kNo_ZeroInitialized)
+        {
+            fSubset.setEmpty();
+        }
 
         ZeroInitialized fZeroInitialized;
+        /**
+         *  If not empty, represents a subset of the original image to decode.
+         *
+         *  Must intersect with the bounds returned by getInfo().
+         *
+         *  If the EncodedFormat is kWEBP_SkEncodedFormat (the only one which
+         *  currently supports subsets), the top and left values must be even.
+         */
+        SkIRect         fSubset;

[scroggo, 2015/07/21 14:53:45]

This might be better as a pointer, where NULL means do not use a subset, and
non-NULL means to use a subset. That way we do not have to initialize/check
empty when it's not used. Thoughts?

[msarett, 2015/07/21 15:19:37]

I would lean towards using a pointer, but I don't feel strongly.

I could see how we might erroneously decide that an empty subset indicates we
should do a no-op decode.

[emmaleer, 2015/07/21 15:24:55]

I think making it a pointer makes more sense. Because why would you want to
create the SkIRect at all if you aren't going to use it? 

     };
 
     /**
@@ -228,6 +259,11 @@ protected:
                                void* pixels, size_t rowBytes, const Options&,
                                SkPMColor ctable[], int* ctableCount) = 0;
 
+    virtual bool onGetValidSubset(SkIRect* /* desiredSubset */) const {
+        // By default, subsets are not supported.
+        return false;
+    }
+
     /**
      *  Override if your codec supports scanline decoding.
      *