Support decoding opaque to *premul

src/codec/SkCodecPriv.h

Index: src/codec/SkCodecPriv.h
diff --git a/src/codec/SkCodecPriv.h b/src/codec/SkCodecPriv.h
index 27e2a63722d8ff095911846ece0d45d46cc7479f..4ae049744c3017b4a1a87da9800845c2404df04c 100644
--- a/src/codec/SkCodecPriv.h
+++ b/src/codec/SkCodecPriv.h
@@ -75,11 +75,14 @@ inline bool is_coord_necessary(int srcCoord, int sampleFactor, int scaledDim) {
 }
 
 inline bool valid_alpha(SkAlphaType dstAlpha, SkAlphaType srcAlpha) {
-    // Check for supported alpha types
+    if (kUnknown_SkAlphaType == dstAlpha) {
+        return false;
+    }
+
     if (srcAlpha != dstAlpha) {
         if (kOpaque_SkAlphaType == srcAlpha) {
-            // If the source is opaque, we must decode to opaque
-            return false;
+            // If the source is opaque, we can support any.
+            return true;
         }
 
         // The source is not opaque
@@ -99,7 +102,8 @@ inline bool valid_alpha(SkAlphaType dstAlpha, SkAlphaType srcAlpha) {
 /*
  * Most of our codecs support the same conversions:
  * - profileType must be the same
- * - opaque only to opaque (and 565 only if opaque)
+ * - opaque to any alpha type
+ * - 565 only if opaque
  * - premul to unpremul and vice versa
  * - always support N32
  * - otherwise match the src color type