Add subsetting to SkScanlineDecoder

src/codec/SkJpegCodec.cpp

Index: src/codec/SkJpegCodec.cpp
diff --git a/src/codec/SkJpegCodec.cpp b/src/codec/SkJpegCodec.cpp
index 04136171fa6fc8df309de88fc5c416acb6344e37..5ae4be9c5d6775bac98afe71c2e906acc5aeb2f3 100644
--- a/src/codec/SkJpegCodec.cpp
+++ b/src/codec/SkJpegCodec.cpp
@@ -177,19 +177,19 @@ SkISize SkJpegCodec::onGetScaledDimensions(float desiredScale) const {
     // support these as well
     unsigned int num;
     unsigned int denom = 8;
-    if (desiredScale > 0.875f) {
+    if (desiredScale >= 0.9375) {
         num = 8;
-    } else if (desiredScale > 0.75f) {
+    } else if (desiredScale >= 0.8125) {
         num = 7;
-    } else if (desiredScale > 0.625f) {
+    } else if (desiredScale >= 0.6875f) {
         num = 6;
-    } else if (desiredScale > 0.5f) {
+    } else if (desiredScale >= 0.5625f) {
         num = 5;
-    } else if (desiredScale > 0.375f) {
+    } else if (desiredScale >= 0.4375f) {
         num = 4;
-    } else if (desiredScale > 0.25f) {
+    } else if (desiredScale >= 0.3125f) {
         num = 3;
-    } else if (desiredScale > 0.125f) {
+    } else if (desiredScale >= 0.1875f) {
         num = 2;
     } else {
         num = 1;
@@ -406,8 +406,7 @@ SkSampler* SkJpegCodec::getSampler(bool createIfNecessary) {
             SkASSERT(false);
     }
 
-    fSwizzler.reset(SkSwizzler::CreateSwizzler(srcConfig, nullptr, info,
-                                               this->options().fZeroInitialized));
+    fSwizzler.reset(SkSwizzler::CreateSwizzler(srcConfig, nullptr, info, this->options()));
     if (!fSwizzler) {
         return nullptr;
     }
@@ -441,6 +440,11 @@ SkCodec::Result SkJpegCodec::onStartScanlineDecode(const SkImageInfo& dstInfo,
         return kInvalidInput;
     }
 
+    // We will need a sampler if we are performing a subset decode
+    if (options.fSubset) {
+        this->getSampler(true);

[scroggo, 2015/10/08 20:16:08]

As mentioned elsewhere, you don't need to have the options stored in
this->options() in order to do this. You can have a helper function that takes
SkImageInfo and options, which can be called here or in getSampler.

[msarett, 2015/10/09 20:03:44]

Yes, there is a better way to do this.

+    }
+
     return kSuccess;
 }
 
@@ -452,7 +456,7 @@ int SkJpegCodec::onGetScanlines(void* dst, int count, size_t rowBytes) {
     // Read rows one at a time
     JSAMPLE* dstRow;
     if (fSwizzler) {
-        // write data to storage row, then sample using swizzler         
+        // write data to storage row, then sample using swizzler
         dstRow = fSrcRow;
     } else {
         // write data directly to dst