Support decoding opaque to *premul

src/codec/SkPngCodec.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 "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkColorTable.h"
@@ skipping 547 matching lines @@
         // We've already read all the scanlines. This is a success.
         return kSuccess;
     }
 
     // read rest of file, and get additional comment and time chunks in info_ptr
     png_read_end(fPng_ptr, fInfo_ptr);
 
     return kSuccess;
 }
 
-uint32_t SkPngCodec::onGetFillValue(SkColorType colorType, SkAlphaType alphaType) const {
+uint32_t SkPngCodec::onGetFillValue(SkColorType colorType) const {
     const SkPMColor* colorPtr = get_color_ptr(fColorTable.get());
     if (colorPtr) {
         return get_color_table_fill_value(colorType, colorPtr, 0);
     }
-    return INHERITED::onGetFillValue(colorType, alphaType);
+    return INHERITED::onGetFillValue(colorType);
 }
 
 // Subclass of SkPngCodec which supports scanline decoding
 class SkPngScanlineDecoder : public SkPngCodec {
 public:
     SkPngScanlineDecoder(const SkImageInfo& srcInfo, SkStream* stream,
             SkPngChunkReader* chunkReader, png_structp png_ptr, png_infop info_ptr, int bitDepth)
         : INHERITED(srcInfo, stream, chunkReader, png_ptr, info_ptr, bitDepth, 1)
         , fSrcRow(nullptr)
     {}
@@ skipping 194 matching lines @@
     }
 
     if (1 == numberPasses) {
         return new SkPngScanlineDecoder(imageInfo, streamDeleter.detach(), chunkReader,
                                         png_ptr, info_ptr, bitDepth);
     }
 
     return new SkPngInterlacedScanlineDecoder(imageInfo, streamDeleter.detach(), chunkReader,
                                               png_ptr, info_ptr, bitDepth, numberPasses);
 }