Make Fl_Image::copy() 'const', including all derived classes

Copying an image does not (and must not) change the original object,
hence copy() should always be 'const'.

This is *necessary* if the given Fl_Image object is 'const'.

FL/Fl_Image.H

@@ -234,7 +234,7 @@ public:
 
   Fl_Image(int W, int H, int D);
   virtual ~Fl_Image();
-  virtual Fl_Image *copy(int W, int H);
+  virtual Fl_Image *copy(int W, int H) const;
   /**
     Creates a copy of the specified image.
     The image should be released when you are done with it.
@@ -247,7 +247,7 @@ public:
     \see Fl_Image::release()
     \see Fl_Image::copy(int w, int h)
   */
-  Fl_Image *copy() { return copy(w(), h()); }
+  Fl_Image *copy() const { return copy(w(), h()); }
   virtual void color_average(Fl_Color c, float i);
   /**
    The inactive() method calls
@@ -340,8 +340,8 @@ public:
   Fl_RGB_Image(const uchar *bits, int W, int H, int D=3, int LD=0);
   Fl_RGB_Image(const Fl_Pixmap *pxm, Fl_Color bg=FL_GRAY);
   virtual ~Fl_RGB_Image();
-  virtual Fl_Image *copy(int W, int H);
-  Fl_Image *copy() { return Fl_Image::copy(); }
+  virtual Fl_Image *copy(int W, int H) const;
+  Fl_Image *copy() const { return Fl_Image::copy(); }
   virtual void color_average(Fl_Color c, float i);
   virtual void desaturate();
   virtual void draw(int X, int Y, int W, int H, int cx=0, int cy=0);

FL/Fl_SVG_Image.H

@@ -147,16 +147,18 @@ private:
   float svg_scaling_(int W, int H);
   void rasterize_(int W, int H);
   virtual void cache_size_(int &width, int &height);
-  void init_(const char *filename, const char *filedata, Fl_SVG_Image *copy_source);
-  Fl_SVG_Image(Fl_SVG_Image *source);
+  void init_(const char *filename, const char *filedata, const Fl_SVG_Image *copy_source);
+  Fl_SVG_Image(const Fl_SVG_Image *source);
 public:
   /** Set this to \c false to allow image re-scaling that alters the image aspect ratio.
    Upon object creation, proportional is set to \c true, and the aspect ratio is kept constant.*/
   bool proportional;
   Fl_SVG_Image(const char *filename, const char *svg_data = NULL);
   virtual ~Fl_SVG_Image();
-  virtual Fl_Image *copy(int W, int H);
-  Fl_Image *copy() { return Fl_Image::copy(); }
+  virtual Fl_Image *copy(int W, int H) const;
+  Fl_Image *copy() const {
+    return Fl_Image::copy();
+  }
   void resize(int width, int height);
   virtual void desaturate();
   virtual void color_average(Fl_Color c, float i);

FL/Fl_Shared_Image.H

@@ -145,8 +145,10 @@ public:
     return this;
   }
 
-  virtual Fl_Image *copy(int W, int H);
-  Fl_Image *copy() { return Fl_Image::copy(); }
+  virtual Fl_Image *copy(int W, int H) const;
+  Fl_Image *copy() const {
+    return Fl_Image::copy();
+  }
   virtual void color_average(Fl_Color c, float i);
   virtual void desaturate();
   virtual void draw(int X, int Y, int W, int H, int cx = 0, int cy = 0);

FL/Fl_Tiled_Image.H

@@ -40,8 +40,10 @@ class FL_EXPORT Fl_Tiled_Image : public Fl_Image {
   Fl_Tiled_Image(Fl_Image *i, int W = 0, int H = 0);
   virtual ~Fl_Tiled_Image();
 
-  virtual Fl_Image *copy(int W, int H);
-  Fl_Image *copy() { return Fl_Image::copy(); }
+  virtual Fl_Image *copy(int W, int H) const;
+  Fl_Image *copy() const {
+    return Fl_Image::copy();
+  }
   virtual void color_average(Fl_Color c, float i);
   virtual void desaturate();
   virtual void draw(int X, int Y, int W, int H, int cx = 0, int cy = 0);