lavfi/vf_idet: replace round and cast by lrint

lrint is faster and conveys the intent better here. It is safe as long int has
at least 32 bits.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/vf_idet.c

@@ -409,7 +409,7 @@ static av_cold int init(AVFilterContext *ctx)
     memset(idet->history, UNDETERMINED, HIST_SIZE);
 
     if( idet->half_life > 0 )
-        idet->decay_coefficient = (uint64_t) round( PRECISION * exp2(-1.0 / idet->half_life) );
+        idet->decay_coefficient = lrint( PRECISION * exp2(-1.0 / idet->half_life) );
     else
         idet->decay_coefficient = PRECISION;