Fix SkSwizzler bug

tests/CodexTest.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Resources.h"
 #include "SkBitmap.h"
 #include "SkCodec.h"
@@ skipping 202 matching lines @@
         const int lines = codec->getScanlines(bm.getAddr(0, 0), 1, 0);
         if (!isIncomplete) {
             REPORTER_ASSERT(r, lines == 1);
         }
         REPORTER_ASSERT(r, codec->getPixels(bm.info(), bm.getPixels(), bm.rowBytes())
                 == expectedResult);
         REPORTER_ASSERT(r, codec->getScanlines(bm.getAddr(0, 0), 1, 0)
                 == 0);
         REPORTER_ASSERT(r, codec->skipScanlines(1)
                 == 0);
+
+        // Test partial scanline decodes
+        if (supports_scaled_codec(path) && info.width() >= 3) {
+            SkCodec::Options options;
+            int width = info.width();
+            int height = info.height();
+            SkIRect subset = SkIRect::MakeXYWH(2 * (width / 3), 0, width / 3, height);
+            options.fSubset = ⊂
+
+            const SkCodec::Result partialStartResult = codec->startScanlineDecode(info, &options,
+                    nullptr, nullptr);
+            REPORTER_ASSERT(r, partialStartResult == SkCodec::kSuccess);
+
+            for (int y = 0; y < height; y++) {
+                const int lines = codec->getScanlines(bm.getAddr(0, y), 1, 0);
+                if (!isIncomplete) {
+                    REPORTER_ASSERT(r, 1 == lines);
+                }
+            }
+        }
     } else {
         REPORTER_ASSERT(r, startResult == SkCodec::kUnimplemented);
     }
 
     // The rest of this function tests decoding subsets, and will decode an arbitrary number of
     // random subsets.
     // Do not attempt to decode subsets of an image of only once pixel, since there is no
     // meaningful subset.
     if (size.width() * size.height() == 1) {
         return;
@@ skipping 356 matching lines @@
     REPORTER_ASSERT(r, SkCodec::kInvalidParameters == result);
     result = decoder->startScanlineDecode(
         decoder->getInfo().makeColorType(kIndex_8_SkColorType));
     REPORTER_ASSERT(r, SkCodec::kInvalidParameters == result);
 }
 
 DEF_TEST(Codec_Params, r) {
     test_invalid_parameters(r, "index8.png");
     test_invalid_parameters(r, "mandrill.wbmp");
 }