mandelbrot: increase default maxiter

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vsrc_mandelbrot.c

@@ -67,7 +67,7 @@ static av_cold int init(AVFilterContext *ctx, const char *args, void *opaque)
     AVRational frame_rate_q;
     int ret;
 
-    mb->maxiter=1024;
+    mb->maxiter=4096;
     mb->start_x=-0.743643887037158704752191506114774;
     mb->start_y=-0.131825904205311970493132056385139;
     mb->start_scale=3.0;