swscale/utils: replace pow by exp2

exp2 is a faster function.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libswscale/utils.c

@@ -45,6 +45,7 @@
 #include "libavutil/cpu.h"
 #include "libavutil/imgutils.h"
 #include "libavutil/intreadwrite.h"
+#include "libavutil/libm.h"
 #include "libavutil/mathematics.h"
 #include "libavutil/opt.h"
 #include "libavutil/pixdesc.h"
@@ -451,7 +452,7 @@ static av_cold int initFilter(int16_t **outFilter, int32_t **filterPos,
                     coeff *= fone >> (30 + 16);
                 } else if (flags & SWS_GAUSS) {
                     double p = param[0] != SWS_PARAM_DEFAULT ? param[0] : 3.0;
-                    coeff = (pow(2.0, -p * floatd * floatd)) * fone;
+                    coeff = exp2(-p * floatd * floatd) * fone;
                 } else if (flags & SWS_SINC) {
                     coeff = (d ? sin(floatd * M_PI) / (floatd * M_PI) : 1.0) * fone;
                 } else if (flags & SWS_LANCZOS) {