Clean up unused or overly complex modes in SkSwizzler

src/codec/SkCodec_libpng.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_libpng.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 295 matching lines @@
             skColorType = kIndex_8_SkColorType;
             skAlphaType = has_transparency_in_tRNS(png_ptr, info_ptr) ?
                     kUnpremul_SkAlphaType : kOpaque_SkAlphaType;
             break;
         case PNG_COLOR_TYPE_RGB:
             if (has_transparency_in_tRNS(png_ptr, info_ptr)) {
                 //convert to RGBA with tranparency information in tRNS chunk if it exists
                 png_set_tRNS_to_alpha(png_ptr);
                 skAlphaType = kUnpremul_SkAlphaType;
             } else {
-                //convert to RGBA with Opaque Alpha 
-                png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);
                 skAlphaType = kOpaque_SkAlphaType;
             }
             skColorType = kN32_SkColorType;
             break;
         case PNG_COLOR_TYPE_GRAY:
             if (has_transparency_in_tRNS(png_ptr, info_ptr)) {
                 //FIXME: support gray with alpha as a color type
                 //convert to RGBA if there is transparentcy info in the tRNS chunk
                 png_set_tRNS_to_alpha(png_ptr);
                 png_set_gray_to_rgb(png_ptr);
@@ skipping 92 matching lines @@
             if (!this->decodePalette(kPremul_SkAlphaType == requestedInfo.alphaType(),
                     ctableCount)) {
                 return kInvalidInput;
             }
             break;
         case kGray_8_SkColorType:
             fSrcConfig = SkSwizzler::kGray;
             break; 
         case kN32_SkColorType:
             if (this->getInfo().alphaType() == kOpaque_SkAlphaType) {
-                    fSrcConfig = SkSwizzler::kRGBX;
+                    fSrcConfig = SkSwizzler::kRGB;
                 } else {
                     fSrcConfig = SkSwizzler::kRGBA;
             }
             break;
         default:
             //would have exited before now if the colorType was supported by png
             SkASSERT(false);
     }  
 
     // Copy the color table to the client if they request kIndex8 mode
@@ skipping 339 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);
 }