use log2(scale) to compute mip level

include/core/SkFloatingPoint.h

Index: include/core/SkFloatingPoint.h
diff --git a/include/core/SkFloatingPoint.h b/include/core/SkFloatingPoint.h
index 1003b80b4f1109db3730c54d0b76ede4ae865703..9fb343261b2b1278bbcba14b4f38aba88d9edb92 100644
--- a/include/core/SkFloatingPoint.h
+++ b/include/core/SkFloatingPoint.h
@@ -91,6 +91,16 @@ static inline float sk_float_copysign(float x, float y) {
     #define sk_float_log(x)         logf(x)
 #endif
 
+// can't find log2f on android, but maybe that just a tool bug?
+#ifdef SK_BUILD_FOR_ANDROID
+    static inline float sk_float_log2(float x) {
+        const double inv_ln_2 = 1.44269504088896;
+        return (float)(log(x) * inv_ln_2);
+    }
+#else
+    #define sk_float_log2(x)        log2f(x)
+#endif
+
 #ifdef SK_BUILD_FOR_WIN
     #define sk_float_isfinite(x)    _finite(x)
     #define sk_float_isnan(x)       _isnan(x)