Add reallyHasAlpha() to SkAndroidCodec

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index 67ccdb241c3225e35e09ef2b75d1fd60bbd64a6b..02d4469ba66049aae9aeac86b7c233e67a854b32 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -486,7 +486,13 @@ protected:
         return false;
     }
 
-    virtual bool onReallyHasAlpha() const { return false; }
+    virtual bool onReallyHasAlpha() const {
+        // Opaque images never have alpha.  If the image indicates that it
+        // does have alpha, by default, we will assume this to be true.
+        // Subclasses may override this function if they intend to verify
+        // that the image actually has alpha.
+        return kOpaque_SkAlphaType != this->getInfo().alphaType();

[scroggo, 2015/12/10 17:02:32]

SkWebpCodec's implementation looks like this (and maybe others? I have not
checked). Should we stop them from overriding to do the same thing?

I wonder if it could be written like the following:

bool reallyHasAlpha() const {
  return this->getInfo().alphaType() != kOpaque && this->onReallyHasAlpha();
}

(sorry for the switched operands for !=; I don't have a preference there...)

[msarett, 2015/12/10 17:41:18]

Yes I think so.  Or we should make this pure virtual.  Only png and webp
remember to override this right now.

So on gif for example, we would have always (sometimes incorrectly) reported
that the image doesn't have alpha.
sgtm

[scroggo, 2015/12/10 18:29:42]

Yeah, I think we have a note about that in skbug.com/3582

+    }
 
     /**
      *  If the stream was previously read, attempt to rewind.