lavfi/vsrc_mandelbrot: replace round by lrint

lrint is at least as fast, and is more accurate.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/vsrc_mandelbrot.c

@@ -183,7 +183,7 @@ static void fill_from_cache(AVFilterContext *ctx, uint32_t *color, int *in_cidx,
         int x;
         if(p->p[1] > py)
             break;
-        x= round((p->p[0] - s->start_x) / scale + s->w/2);
+        x= lrint((p->p[0] - s->start_x) / scale + s->w/2);
         if(x<0 || x >= s->w)
             continue;
         if(color) color[x] = p->val;