use log2(scale) to compute mip level

include/core/SkScalar.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 SkScalar_DEFINED
 #define SkScalar_DEFINED
 
@@ skipping 40 matching lines @@
 #define SkScalarPow(b, e)           sk_float_pow(b, e)
 
 #define SkScalarSin(radians)        (float)sk_float_sin(radians)
 #define SkScalarCos(radians)        (float)sk_float_cos(radians)
 #define SkScalarTan(radians)        (float)sk_float_tan(radians)
 #define SkScalarASin(val)           (float)sk_float_asin(val)
 #define SkScalarACos(val)           (float)sk_float_acos(val)
 #define SkScalarATan2(y, x)         (float)sk_float_atan2(y,x)
 #define SkScalarExp(x)              (float)sk_float_exp(x)
 #define SkScalarLog(x)              (float)sk_float_log(x)
+#define SkScalarLog2(x)             (float)sk_float_log2(x)
 
 #else   // SK_SCALAR_IS_DOUBLE
 
 typedef double SkScalar;
 
 #define SK_Scalar1                  1.0
 #define SK_ScalarHalf               0.5
 #define SK_ScalarSqrt2              1.414213562373095
 #define SK_ScalarPI                 3.141592653589793
 #define SK_ScalarTanPIOver8         0.4142135623731
@@ skipping 22 matching lines @@
 #define SkScalarPow(b, e)           pow(b, e)
 
 #define SkScalarSin(radians)        sin(radians)
 #define SkScalarCos(radians)        cos(radians)
 #define SkScalarTan(radians)        tan(radians)
 #define SkScalarASin(val)           asin(val)
 #define SkScalarACos(val)           acos(val)
 #define SkScalarATan2(y, x)         atan2(y,x)
 #define SkScalarExp(x)              exp(x)
 #define SkScalarLog(x)              log(x)
+#define SkScalarLog2(x)             log2(x)
 
 #endif
 
 //////////////////////////////////////////////////////////////////////////////////////////////////
 
 #define SkIntToScalar(x)        static_cast<SkScalar>(x)
 #define SkScalarTruncToInt(x)   static_cast<int>(x)
 
 #define SkScalarToFloat(x)      static_cast<float>(x)
 #define SkFloatToScalar(x)      static_cast<SkScalar>(x)
@@ skipping 128 matching lines @@
     SkASSERT(n >= 0);
     for (int i = 0; i < n; ++i) {
         if (a[i] != b[i]) {
             return false;
         }
     }
     return true;
 }
 
 #endif