lavfi/mandelbrot: fix speedloss with default config after morphing introduction.

Morphing was introduced in 0d6e5a171 and forced cos/sin computations
with some mult all the time. This commit makes sure these are computed
only when morphing is enabled.

libavfilter/vsrc_mandelbrot.c

@@ -271,21 +271,27 @@ static void draw_mandelbrot(AVFilterContext *ctx, uint32_t *color, int linesize,
         for(x=0; x<mb->w; x++){
             float av_uninit(epsilon);
             const double cr=mb->start_x+scale*(x-mb->w/2);
-            double zr=cr + cos(pts * mb->morphxf) * mb->morphamp;
-            double zi=ci + sin(pts * mb->morphyf) * mb->morphamp;
+            double zr=cr;
+            double zi=ci;
             uint32_t c=0;
             double dv= mb->dither / (double)(1LL<<32);
             mb->dither= mb->dither*1664525+1013904223;
 
             if(color[x + y*linesize] & 0xFF000000)
                 continue;
-            if(!mb->morphamp && interpol(mb, color, x, y, linesize)){
+            if(!mb->morphamp){
+                if(interpol(mb, color, x, y, linesize)){
+                //TODO: reindent
                 if(next_cidx < mb->cache_allocated){
                     mb->next_cache[next_cidx  ].p[0]= cr;
                     mb->next_cache[next_cidx  ].p[1]= ci;
                     mb->next_cache[next_cidx++].val = color[x + y*linesize];
                 }
                 continue;
+                }
+            }else{
+                zr += cos(pts * mb->morphxf) * mb->morphamp;
+                zi += sin(pts * mb->morphyf) * mb->morphamp;
             }
 
             use_zyklus= (x==0 || mb->inner!=BLACK ||color[x-1 + y*linesize] == 0xFF000000);