Split SkBmpCodec into three separate classes

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 379 matching lines @@
 ///////////////////////////////////////////////////////////////////////////////
 // Getting the pixels
 ///////////////////////////////////////////////////////////////////////////////
 
 static bool conversion_possible(const SkImageInfo& dst, const SkImageInfo& src) {
     // TODO: Support other conversions
     if (dst.profileType() != src.profileType()) {
         return false;
     }
 
-    // Check for supported alpha types
-    if (src.alphaType() != dst.alphaType()) {
-        if (kOpaque_SkAlphaType == src.alphaType()) {
-            // If the source is opaque, we must decode to opaque
-            return false;
-        }
+    // Ensure the alpha type is valid
+    if (!valid_alpha(dst.alphaType(), src.alphaType())) {
+        return false;
+    }
 
-        // The source is not opaque
-        switch (dst.alphaType()) {
-            case kPremul_SkAlphaType:
-            case kUnpremul_SkAlphaType:
-                // The source is not opaque, so either of these is okay
-                break;
-            default:
-                // We cannot decode a non-opaque image to opaque (or unknown)
-                return false;
-        }
-    }
     // Check for supported color types
     switch (dst.colorType()) {
         case kN32_SkColorType:
             return true;
         default:
             return dst.colorType() == src.colorType();
     }
 }
 
 SkCodec::Result SkPngCodec::initializeSwizzler(const SkImageInfo& requestedInfo,
@@ skipping 322 matching lines @@
 
     SkASSERT(codec->fNumberPasses != INVALID_NUMBER_PASSES);
     if (codec->fNumberPasses > 1) {
         // interlaced image
         return SkNEW_ARGS(SkPngInterlacedScanlineDecoder, (dstInfo, codec.detach()));
     }
 
     return SkNEW_ARGS(SkPngScanlineDecoder, (dstInfo, codec.detach()));
 }