mandelbrot: make 2 variables const

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

libavfilter/vsrc_mandelbrot.c

@@ -121,8 +121,8 @@ static void draw_mandelbrot(AVFilterContext *ctx, uint32_t *color, int linesize,
 
     for(y=0; y<mb->h; y++){
         for(x=0; x<mb->w; x++){
-            double cr=mb->start_x+mb->start_scale*x;
-            double ci=mb->start_y+mb->start_scale*y;
+            const double cr=mb->start_x+mb->start_scale*x;
+            const double ci=mb->start_y+mb->start_scale*y;
             double zr=cr;
             double zi=ci;
             uint32_t c=0;