Add an option on SkImageDecoder to skip writing 0s.

src/images/SkImageDecoder_libjpeg.cpp

Index: src/images/SkImageDecoder_libjpeg.cpp
diff --git a/src/images/SkImageDecoder_libjpeg.cpp b/src/images/SkImageDecoder_libjpeg.cpp
index d50e4eb015689335b3910c97f58104bb41dc14c7..bf596d73f05edae4796789057627d200e36108c7 100644
--- a/src/images/SkImageDecoder_libjpeg.cpp
+++ b/src/images/SkImageDecoder_libjpeg.cpp
@@ -587,7 +587,7 @@ bool SkJPEGImageDecoder::onDecode(SkStream* stream, SkBitmap* bm, Mode mode) {
         return return_false(cinfo, *bm, "jpeg colorspace");
     }
 
-    if (!sampler.begin(bm, sc, this->getDitherImage())) {
+    if (!sampler.begin(bm, sc, *this)) {
         return return_false(cinfo, *bm, "sampler.begin");
     }
 
@@ -826,7 +826,7 @@ bool SkJPEGImageDecoder::onDecodeSubset(SkBitmap* bm, const SkIRect& region) {
         return return_false(*cinfo, *bm, "jpeg colorspace");
     }
 
-    if (!sampler.begin(&bitmap, sc, this->getDitherImage())) {
+    if (!sampler.begin(&bitmap, sc, *this)) {
         return return_false(*cinfo, bitmap, "sampler.begin");
     }