avcodec/aacenc_is: replace pow(x, 0.75) by x/sqrtf(sqrtf(x))

This is quite an accurate approximation; testing shows ~ 2ulp error in
the floating point result. Tested with FATE.

Alternatively, if one wants "full accuracy", one can use powf, or sqrt
instead of sqrtf. With powf, one gets 1 ulp error (theoretically should be 0, as
0.75 is exactly representable) on GNU libm, with sqrt, 0 ulp error.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

Abstracted into pos_pow34 utility function
Signed-off-by: Claudio Freire <klaussfreire@gmail.com>

libavcodec/aacenc_is.c

@@ -54,7 +54,7 @@ struct AACISError ff_aac_is_encoding_err(AACEncContext *s, ChannelElement *cpe,
         FFPsyBand *band0 = &s->psy.ch[s->cur_channel+0].psy_bands[(w+w2)*16+g];
         FFPsyBand *band1 = &s->psy.ch[s->cur_channel+1].psy_bands[(w+w2)*16+g];
         int is_band_type, is_sf_idx = FFMAX(1, sce0->sf_idx[w*16+g]-4);
-        float e01_34 = phase*pow(ener1/ener0, 3.0/4.0);
+        float e01_34 = phase*pos_pow34(ener1/ener0);
         float maxval, dist_spec_err = 0.0f;
         float minthr = FFMIN(band0->threshold, band1->threshold);
         for (i = 0; i < sce0->ics.swb_sizes[g]; i++)

libavcodec/aacenc_utils.h

@@ -45,6 +45,11 @@ static inline void abs_pow34_v(float *out, const float *in, const int size)
     }
 }
 
+static inline float pos_pow34(float a)
+{
+    return sqrtf(a * sqrtf(a));
+}
+
 /**
  * Quantize one coefficient.
  * @return absolute value of the quantized coefficient