change setAlphaType to not modify the pixelref's genID

include/core/SkBitmap.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkBitmap_DEFINED
 #define SkBitmap_DEFINED
 
@@ skipping 113 matching lines @@
 
     SkAlphaType alphaType() const { return (SkAlphaType)fAlphaType; }
 
     /**
      *  Set the bitmap's alphaType, returning true on success. If false is
      *  returned, then the specified new alphaType is incompatible with the
      *  Config, and the current alphaType is unchanged.
      *
      *  Note: this changes the alphatype for the underlying pixels, which means
      *  that all bitmaps that might be sharing (subsets of) the pixels will
-     *  be affected. This is an expensive change for some backends (e.g. GPU)
-     *  since changing the alphatype can invalidate internal caches. Thus this
-     *  call should only be made if it is associated with real changes to the
-     *  pixel data.
+     *  be affected.
      */
     bool setAlphaType(SkAlphaType);
 
     /** Return the address of the pixels for this SkBitmap.
     */
     void* getPixels() const { return fPixels; }
 
     /** Return the byte size of the pixels, based on the height and rowBytes.
         Note this truncates the result to 32bits. Call getSize64() to detect
         if the real size exceeds 32bits.
@@ skipping 730 matching lines @@
 
 inline SkPMColor SkBitmap::getIndex8Color(int x, int y) const {
     SkASSERT(fPixels);
     SkASSERT(fConfig == kIndex8_Config);
     SkASSERT((unsigned)x < fWidth && (unsigned)y < fHeight);
     SkASSERT(fColorTable);
     return (*fColorTable)[*((const uint8_t*)fPixels + y * fRowBytes + x)];
 }
 
 #endif