Add subsetting to SkScanlineDecoder

src/codec/SkCodec.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 "SkBmpCodec.h"
 #include "SkCodec.h"
 #include "SkData.h"
@@ skipping 202 matching lines @@
 
     if (!this->rewindIfNeeded()) {
         return kCouldNotRewind;
     }
 
     // Set options.
     Options optsStorage;
     if (nullptr == options) {
         options = &optsStorage;
     } else if (options->fSubset) {
-        SkIRect subset(*options->fSubset);
-        if (!this->onGetValidSubset(&subset) || subset != *options->fSubset) {
-            // FIXME: How to differentiate between not supporting subset at all
-            // and not supporting this particular subset?
-            return kUnimplemented;
+        SkIRect size = SkIRect::MakeSize(dstInfo.dimensions());
+        if (!size.contains(*options->fSubset)) {
+            return kInvalidInput;
+        }
+
+        // We only support subsetting in the x-dimension for scanline decoder.
+        // Subsetting in the y-dimension can be accomplished using skipScanlines().
+        if (options->fSubset->top() != 0 || options->fSubset->height() != dstInfo.height()) {
+            return kInvalidInput;
         }
     }
 
     // FIXME: Support subsets somehow?
     if (!this->dimensionsSupported(dstInfo.dimensions())) {
         return kInvalidScale;
     }
 
     const Result result = this->onStartScanlineDecode(dstInfo, *options, ctable, ctableCount);
     if (result != SkCodec::kSuccess) {
@@ skipping 101 matching lines @@
             SkASSERT(1 == linesRequested || this->getInfo().height() == linesRequested);
             const SkImageInfo fillInfo = info.makeWH(info.width(), 1);
             for (int srcY = linesDecoded; srcY < linesRequested; srcY++) {
                 fillDst = SkTAddOffset<void>(dst, this->outputScanline(srcY) * rowBytes);
                 fill_proc(fillInfo, fillDst, rowBytes, fillValue, zeroInit, sampler);
             }
             break;
         }
     }
 }