Add faster powf approximation for Android.

src/core/SkPaint.cpp

 /*
  * 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.
  */
 
 #include "SkPaint.h"
 #include "SkAnnotation.h"
 #include "SkAutoKern.h"
@@ skipping 1762 matching lines @@
             // filter down the luminance color to a finite number of bits
             SkColor color = rec->getLuminanceColor();
             rec->setLuminanceColor(SkMaskGamma::CanonicalColor(color));
             break;
         }
         case SkMask::kA8_Format: {
             // filter down the luminance to a single component, since A8 can't
             // use per-component information
 
             SkColor color = rec->getLuminanceColor();
-            U8CPU lum = SkColorSpaceLuminance::computeLuminance(rec->getPaintGamma(), color);
-            //If we are asked to look like LCD, look like LCD.
-            if (!(rec->fFlags & SkScalerContext::kGenA8FromLCD_Flag)) {
-                // HACK: Prevents green from being pre-blended as white.
-                lum -= ((255 - lum) * lum) / 255;
-            }
-
+            U8CPU lum = SkComputeLuminance(SkColorGetR(color),
+                                           SkColorGetG(color),
+                                           SkColorGetB(color));
+                                           
             // reduce to our finite number of bits
             color = SkColorSetRGB(lum, lum, lum);
             rec->setLuminanceColor(SkMaskGamma::CanonicalColor(color));
             break;
         }
         case SkMask::kBW_Format:
             // No need to differentiate gamma if we're BW
             rec->ignorePreBlend();
             break;
     }
@@ skipping 948 matching lines @@
     }
 #ifdef SK_BUILD_FOR_ANDROID
     if (dirty & kPaintOptionsAndroid_DirtyBit) {
         SkPaintOptionsAndroid options;
         options.unflatten(buffer);
         paint->setPaintOptionsAndroid(options);
     }
 #endif
     SkASSERT(dirty == paint->fDirtyBits);
 }