Fix FTBFS reported in issue #67

Signed-off-by: Michele Calgaro <michele.calgaro@yahoo.it>

chalk/plugins/filters/cimg/CImg.h

@@ -11097,9 +11097,9 @@ namespace cimg_library {
   
     // INNER CLASS used by function CImg<>::draw_fill()
     template<typename T1,typename T2> struct _draw_fill {
-      const T1 *const color;
-      const float sigma,opacity;
-      const CImg<T1> value;
+      const T1 *color;
+      float sigma,opacity;
+      CImg<T1> value;
       CImg<T2> region;      
 
       _draw_fill(const CImg<T1>& img,const int x,const int y,const int z,