avfilter/vsrc_mandelbrot: avoid sqrt for epsilon calculation

This rewrites into a similar expression avoiding sqrt. Similarity is
assured since sqrt(x^2 + y^2)/(x+y) lies in [1/sqrt(2), 1] for x, y > 0.

Tested on x86-64, Haswell, GNU/Linux.
Command:
ffmpeg -f lavfi -i mandelbrot -f null -

old (draw_mandelbrot):
277625266 decicycles in draw_mandelbrot,     256 runs,      0 skips
304527322 decicycles in draw_mandelbrot,     512 runs,      0 skips
377593582 decicycles in draw_mandelbrot,    1024 runs,      0 skips
338539499 decicycles in draw_mandelbrot,    2048 runs,      0 skips
583630357 decicycles in draw_mandelbrot,    4096 runs,      0 skips

new (draw_mandelbrot):
274635709 decicycles in draw_mandelbrot,     256 runs,      0 skips
300287046 decicycles in draw_mandelbrot,     512 runs,      0 skips
371819935 decicycles in draw_mandelbrot,    1024 runs,      0 skips
336663765 decicycles in draw_mandelbrot,    2048 runs,      0 skips
581851016 decicycles in draw_mandelbrot,    4096 runs,      0 skips
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/vsrc_mandelbrot.c

@@ -291,7 +291,7 @@ static void draw_mandelbrot(AVFilterContext *ctx, uint32_t *color, int linesize,
 
             use_zyklus= (x==0 || s->inner!=BLACK ||color[x-1 + y*linesize] == 0xFF000000);
             if(use_zyklus)
-                epsilon= scale*1*sqrt(SQR(x-s->w/2) + SQR(y-s->h/2))/s->w;
+                epsilon= scale*(abs(x-s->w/2) + abs(y-s->h/2))/s->w;
 
 #define Z_Z2_C(outr,outi,inr,ini)\
             outr= inr*inr - ini*ini + cr;\