Implement canvas's globalCompositeOperation lighter properly....

skia/include/SkPorterDuff.h

 /*
  * Copyright (C) 2006 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
@@ skipping 30 matching lines @@
         kDstIn_Mode,    //!< [Sa * Da, Sa * Dc]
         kSrcOut_Mode,   //!< [Sa * (1 - Da), Sc * (1 - Da)]
         kDstOut_Mode,   //!< [Da * (1 - Sa), Dc * (1 - Sa)]
         kSrcATop_Mode,  //!< [Da, Sc * Da + (1 - Sa) * Dc]
         kDstATop_Mode,  //!< [Sa, Sa * Dc + Sc * (1 - Da)]
         kXor_Mode,      //!< [Sa + Da - 2 * Sa * Da, Sc * (1 - Da) + (1 - Sa) * Dc]
         kDarken_Mode,   //!< [Sa + Da - Sa*Da, Sc*(1 - Da) + Dc*(1 - Sa) + min(Sc, Dc)]
         kLighten_Mode,  //!< [Sa + Da - Sa*Da, Sc*(1 - Da) + Dc*(1 - Sa) + max(Sc, Dc)]
         kMultiply_Mode, //!< [Sa * Da, Sc * Dc]
         kScreen_Mode,   //!< [Sa + Da - Sa * Da, Sc + Dc - Sc * Dc]
+        kAdd_Mode,      //!< Saturate(S + D)
 
         kModeCount
     };
     /** Return an SkXfermode object for the specified mode.
     */
     static SkXfermode* CreateXfermode(Mode mode);
     
     /** Return a function pointer to a routine that applies the specified
         porter-duff transfer mode.
     */
     static SkXfermodeProc GetXfermodeProc(Mode mode);
     
     /** Return a function pointer to a routine that applies the specified
         porter-duff transfer mode and srcColor to a 16bit device color. Note,
         if the mode+srcColor might return a non-opaque color, then there is not
         16bit proc, and this will return NULL.
     */
     static SkXfermodeProc16 GetXfermodeProc16(Mode mode, SkColor srcColor);
     
     /** If the specified xfermode advertises itself as one of the porterduff
         modes (via SkXfermode::Coeff), return true and if not null, set mode
         to the corresponding porterduff mode. If it is not recognized as a one,
         return false and ignore the mode parameter.
     */
     static bool IsMode(SkXfermode*, Mode* mode);
 };
 
 #endif