avcodec/snowenc: use log2 instead of log() / log(2...)

This is likely more precise and conveys the intent better.
The expression has also been accordingly simplified.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/snowenc.c

@@ -1547,7 +1547,7 @@ static void calculate_visual_weight(SnowContext *s, Plane *p){
                 }
             }
 
-            b->qlog= (int)(log(352256.0/sqrt(error)) / log(pow(2.0, 1.0/QROOT))+0.5);
+            b->qlog= (int)(QROOT * log2(352256.0/sqrt(error)) + 0.5);
         }
     }
 }