Add the ability to decode a subset to SkCodec.

dm/DMSrcSink.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 DMSrcSink_DEFINED
 #define DMSrcSink_DEFINED
 
@@ skipping 81 matching lines @@
     skiagm::GMRegistry::Factory fFactory;
 };
 
 class CodecSrc : public Src {
 public:
     enum Mode {
         kNormal_Mode,
         kScanline_Mode,
         kScanline_Subset_Mode,
         kStripe_Mode, // Tests the skipping of scanlines
+        kSubset_Mode, // For codecs that support subsets directly.
     };
     enum DstColorType {
         kGetFromCanvas_DstColorType,
         kIndex8_Always_DstColorType,
         kGrayscale_Always_DstColorType,
     };
     CodecSrc(Path, Mode, DstColorType, float);
 
     Error draw(SkCanvas*) const override;
     SkISize size() const override;
@@ skipping 174 matching lines @@
 
 class ViaTwice : public Via {
 public:
     explicit ViaTwice(Sink* sink) : Via(sink) {}
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
 };
 
 }  // namespace DM
 
 #endif//DMSrcSink_DEFINED