Support decoding opaque images to unpremul/premul

src/codec/SkCodecPriv.h

 /*
  * Copyright 2015 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCodecPriv_DEFINED
 #define SkCodecPriv_DEFINED
 
@@ skipping 15 matching lines @@
 #define UPDATE_RESULT_ALPHA(alpha)              \
     zeroAlpha |= (alpha);                       \
     maxAlpha  &= (alpha);
 
 #define COMPUTE_RESULT_ALPHA                    \
     SkSwizzler::GetResult(zeroAlpha, maxAlpha);
 
 static inline bool valid_alpha(SkAlphaType dstAlpha, SkAlphaType srcAlpha) {
     // Check for supported alpha types
     if (srcAlpha != dstAlpha) {
-        if (kOpaque_SkAlphaType == srcAlpha) {
-            // If the source is opaque, we must decode to opaque
+        if (kOpaque_SkAlphaType == dstAlpha) {
             return false;
         }
 
-        // The source is not opaque
         switch (dstAlpha) {
+            case kOpaque_SkAlphaType:
+                // If the source is not opaque, we cannot decode to opaque
+                return false;
             case kPremul_SkAlphaType:
             case kUnpremul_SkAlphaType:
-                // The source is not opaque, so either of these is okay
+                // Opaque can trivially be treated as both, so no conversion is
+                // actually necessary.
+                // Switching between premul and unpremul is supported.
                 break;
             default:
-                // We cannot decode a non-opaque image to opaque (or unknown)
+                // We cannot decode a non-opaque image to opaque (or unknown
                 return false;
         }
     }
     return true;
 }
 
 /*
  * If there is a color table, get a pointer to the colors, otherwise return NULL
  */
 static const SkPMColor* get_color_ptr(SkColorTable* colorTable) {
@@ skipping 79 matching lines @@
 #endif
 }
 
 #ifdef SK_PRINT_CODEC_MESSAGES
     #define SkCodecPrintf SkDebugf
 #else
     #define SkCodecPrintf(...)
 #endif
 
 #endif // SkCodecPriv_DEFINED