Add the ability to decode a subset to SkCodec.

src/codec/SkCodec_libbmp.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 "SkCodec_libbmp.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 567 matching lines @@
                                         int* inputColorCount) {
     // Check for proper input and output formats
     SkCodec::RewindState rewindState = this->rewindIfNeeded();
     if (rewindState == kCouldNotRewind_RewindState) {
         return kCouldNotRewind;
     } else if (rewindState == kRewound_RewindState) {
         if (!ReadHeader(this->stream(), fIsIco, NULL)) {
             return kCouldNotRewind;
         }
     }
+    if (opts.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    }
     if (dstInfo.dimensions() != this->getInfo().dimensions()) {
         SkCodecPrintf("Error: scaling not supported.\n");
         return kInvalidScale;
     }
     if (!conversion_possible(dstInfo, this->getInfo())) {
         SkCodecPrintf("Error: cannot convert input type to output type.\n");
         return kInvalidConversion;
     }
 
     // Create the color table if necessary and prepare the stream for decode
@@ skipping 623 matching lines @@
                 uint32_t alphaBit =
                         (srcBuffer.get()[quotient] >> shift) & 0x1;
                 dstRow[x] &= alphaBit - 1;
             }
         }
     }
 
     // Finished decoding the entire image
     return kSuccess;
 }