Add an option on SkImageDecoder to skip writing 0s.

src/images/SkImageDecoder_libbmp.cpp

Index: src/images/SkImageDecoder_libbmp.cpp
diff --git a/src/images/SkImageDecoder_libbmp.cpp b/src/images/SkImageDecoder_libbmp.cpp
index 3eb181f781bc9d699d83a4c656003e3895fac700..2283dbf51d480289427b71cbcc12559026bddd15 100644
--- a/src/images/SkImageDecoder_libbmp.cpp
+++ b/src/images/SkImageDecoder_libbmp.cpp
@@ -146,7 +146,7 @@ bool SkBMPImageDecoder::onDecode(SkStream* stream, SkBitmap* bm, Mode mode) {
 
     SkAutoLockPixels alp(*bm);
 
-    if (!sampler.begin(bm, SkScaledBitmapSampler::kRGB, getDitherImage())) {
+    if (!sampler.begin(bm, SkScaledBitmapSampler::kRGB, *this)) {
         return false;
     }