Add an option on SkImageDecoder to skip writing 0s.

tools/skimage_main.cpp

Index: tools/skimage_main.cpp
diff --git a/tools/skimage_main.cpp b/tools/skimage_main.cpp
index 7cd15e811e0ded6b566de23563888f5918c915c4..d98a6191c30a3201f689f0f18d9e2dd061ef734d 100644
--- a/tools/skimage_main.cpp
+++ b/tools/skimage_main.cpp
@@ -30,6 +30,7 @@ DEFINE_string(readExpectationsPath, "", "Path to read JSON expectations from.");
 DEFINE_bool(reencode, true, "Reencode the images to test encoding.");
 DEFINE_bool(testSubsetDecoding, true, "Test decoding subsets of images.");
 DEFINE_string2(writePath, w, "",  "Write rendered images into this directory.");
+DEFINE_bool(skip, false, "Skip writing zeroes.");
 
 struct Format {
     SkImageEncoder::Type    fType;
@@ -383,6 +384,7 @@ static void decodeFileAndWrite(const char srcPath[], const SkString* writePath)
 
     SkAutoTDelete<SkImageDecoder> ad(codec);
 
+    codec->setSkipWritingZeroes(FLAGS_skip);
     stream.rewind();
 
     // Create a string representing just the filename itself, for use in json expectations.