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Update the log2 approximation to accomodate the increased bit precision from bd0610d21b28b76095a49e601df44fb792b96369. This requires another term in the power series to reduce the error to the level of truncation (peak absolute error 0.621785). Also refactor the rounding bias term so that further changes to DB_SHIFT will leave the result unbiased.

--- a/libcelt/mathops.h

+++ b/libcelt/mathops.h

@@ -306,15 +306,14 @@

 {

    int i;

    celt_word16 n, frac;

-   /*-0.41446   0.96093  -0.33981   0.15600 */

-   /* -0.4144541824871411+32/16384, 0.9590923197873218, -0.3395129038105771,

-       0.16541096501128538 */

-   const celt_word16 C[4] = {-6758, 15715, -5563, 2708};

+   /* -0.41509302963303146, 0.9609890551383969, -0.31836011537636605,

+       0.15530808010959576, -0.08556153059057618 */

+   const celt_word16 C[5] = {-6801+(1<<13-DB_SHIFT), 15746, -5217, 2545, -1401};

    if (x==0)

       return -32767;

    i = celt_ilog2(x);

    n = VSHR32(x,i-15)-32768-16384;

-   frac = ADD16(C[0], MULT16_16_Q15(n, ADD16(C[1], MULT16_16_Q15(n, ADD16(C[2], MULT16_16_Q15(n, (C[3])))))));

+   frac = ADD16(C[0], MULT16_16_Q15(n, ADD16(C[1], MULT16_16_Q15(n, ADD16(C[2], MULT16_16_Q15(n, ADD16(C[3], MULT16_16_Q15(n, C[4]))))))));

    return SHL16(i-13,DB_SHIFT)+SHR16(frac,14-DB_SHIFT);

 }