Support decoding opaque images to unpremul/premul

src/codec/SkCodecPriv.h

Index: src/codec/SkCodecPriv.h
diff --git a/src/codec/SkCodecPriv.h b/src/codec/SkCodecPriv.h
index 2596787b9b7aadcf44819e23849467b137c48cc7..10719c96fc1f68ae7a06cb638a7f20016ad4f925 100644
--- a/src/codec/SkCodecPriv.h
+++ b/src/codec/SkCodecPriv.h
@@ -33,19 +33,22 @@
 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;
         }
     }