Add reallyHasAlpha() to SkAndroidCodec

include/codec/SkCodec.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCodec_DEFINED
 #define SkCodec_DEFINED
 
@@ skipping 468 matching lines @@
     virtual Result onGetPixels(const SkImageInfo& info,
                                void* pixels, size_t rowBytes, const Options&,
                                SkPMColor ctable[], int* ctableCount,
                                int* rowsDecoded) = 0;
 
     virtual bool onGetValidSubset(SkIRect* /* desiredSubset */) const {
         // By default, subsets are not supported.
         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.
      *
      *  If the stream needed to be rewound, call onRewind.
      *  @returns true if the codec is at the right position and can be used.
      *      false if there was a failure to rewind.
      *
      *  This is called by getPixels() and start(). Subclasses may call if they
      *  need to rewind at another time.
@@ skipping 141 matching lines @@
      *  not affect ownership.
      *
      *  Only valid during scanline decoding.
      */
     virtual SkSampler* getSampler(bool /*createIfNecessary*/) { return nullptr; }
 
     friend class SkSampledCodec;
     friend class SkIcoCodec;
 };
 #endif // SkCodec_DEFINED