Support decoding opaque images to unpremul/premul

tests/CodexTest.cpp

Index: tests/CodexTest.cpp
diff --git a/tests/CodexTest.cpp b/tests/CodexTest.cpp
index f52765b171e62ba5217143e5b30e985e1e7969f7..a2412e2f7cd41ea83d81e4eaaace4c124f181048 100644
--- a/tests/CodexTest.cpp
+++ b/tests/CodexTest.cpp
@@ -112,6 +112,11 @@ static void check(skiatest::Reporter* r,
     {
         // Check alpha type conversions
         if (info.alphaType() == kOpaque_SkAlphaType) {
+            // Decoding to either premul or unpremul should succeed and look no different
+            test_info(r, codec, info.makeAlphaType(kUnpremul_SkAlphaType), SkCodec::kSuccess,
+                      &digest);
+            test_info(r, codec, info.makeAlphaType(kPremul_SkAlphaType), SkCodec::kSuccess,
+                      &digest);
         } else {
             // Decoding to opaque should fail
             test_info(r, codec, info.makeAlphaType(kOpaque_SkAlphaType),