Revert parts of and fix other 'constness' changes (#239, #181)

Some of the previous constness changes turned out to be incomplete,
others had to be reverted because some other driver methods could
not be made 'const' - particularly those calling open_display() to
get the requested information.

src/Fl_Screen_Driver.H

@@ -69,7 +69,7 @@ public:
   static char bg2_set;
   static char fg_set;
 
-  virtual float scale(int n) const { return 1; }
+  virtual float scale(int n) { return 1; }
   virtual void scale(int n, float f) {}
   static Fl_Screen_Driver *newScreenDriver();
   // --- display management
@@ -77,13 +77,13 @@ public:
   virtual int visual(int flags);
   // --- screen configuration
   virtual void init() {}
-  virtual int x() const { return 0; }
-  virtual int y() const { return 0; }
-  virtual int w() const { return 800; } // default, override in driver!
-  virtual int h() const { return 600; } // default, override in driver!
+  virtual int x() { return 0; }
+  virtual int y() { return 0; }
+  virtual int w() { return 800; } // default, override in driver!
+  virtual int h() { return 600; } // default, override in driver!
   virtual int screen_count();
   void screen_xywh(int &X, int &Y, int &W, int &H, int mx, int my);
-  virtual void screen_xywh(int &X, int &Y, int &W, int &H, int n) const {
+  virtual void screen_xywh(int &X, int &Y, int &W, int &H, int n) {
     X = 0;
     Y = 0;
     W = 800;
@@ -109,14 +109,14 @@ public:
   virtual void grab(Fl_Window *win) {}
   // --- global colors
   /* the default implementation of parse_color() may be enough */
-  virtual int parse_color(const char *p, uchar &r, uchar &g, uchar &b) const;
+  virtual int parse_color(const char *p, uchar &r, uchar &g, uchar &b);
   virtual void get_system_colors() {}
   /* the default implementation of get_system_scheme() may be enough */
   virtual const char *get_system_scheme();
   // --- global timers
   virtual void add_timeout(double time, Fl_Timeout_Handler cb, void *argp) {}
   virtual void repeat_timeout(double time, Fl_Timeout_Handler cb, void *argp) {}
-  virtual int has_timeout(Fl_Timeout_Handler cb, void *argp) const { return 0; }
+  virtual int has_timeout(Fl_Timeout_Handler cb, void *argp) { return 0; }
   virtual void remove_timeout(Fl_Timeout_Handler cb, void *argp) {}
 
   static int secret_input_character;

src/Fl_Screen_Driver.cxx

@@ -488,7 +488,7 @@ void Fl_Screen_Driver::open_display()
 
 
 // simulation of XParseColor:
-int Fl_Screen_Driver::parse_color(const char* p, uchar& r, uchar& g, uchar& b) const
+int Fl_Screen_Driver::parse_color(const char* p, uchar& r, uchar& g, uchar& b)
 {
   if (*p == '#') p++;
   size_t n = strlen(p);

src/drivers/Cocoa/Fl_Cocoa_Screen_Driver.H

@@ -85,7 +85,7 @@ public:
   virtual void repeat_timeout(double time, Fl_Timeout_Handler cb, void *argp);
   virtual int has_timeout(Fl_Timeout_Handler cb, void *argp);
   virtual void remove_timeout(Fl_Timeout_Handler cb, void *argp);
-  virtual int has_marked_text();
+  virtual int has_marked_text() const;
   virtual void reset_marked_text();
   virtual void insertion_point_location(int x, int y, int height);
   int insertion_point_location(int *px, int *py, int *pheight);

src/drivers/Cocoa/Fl_Cocoa_Screen_Driver.cxx

@@ -210,8 +210,8 @@ void Fl_Cocoa_Screen_Driver::get_system_colors()
 }
 
 
-int Fl_Cocoa_Screen_Driver::has_marked_text() {
-  return true;
+int Fl_Cocoa_Screen_Driver::has_marked_text() const {
+  return 1;
 }
 
 

src/drivers/X11/Fl_X11_Screen_Driver.H

@@ -92,7 +92,7 @@ public:
   virtual int dnd(int unused);
   virtual int compose(int &del);
   virtual void compose_reset();
-  virtual int text_display_can_leak();
+  virtual int text_display_can_leak() const;
   virtual Fl_RGB_Image *read_win_rectangle(int X, int Y, int w, int h, Fl_Window *win, bool may_capture_subwins, bool *did_capture_subwins);
   virtual int get_mouse(int &x, int &y);
   virtual void enable_im();

src/drivers/X11/Fl_X11_Screen_Driver.cxx

@@ -701,7 +701,7 @@ void Fl_X11_Screen_Driver::compose_reset()
   if (fl_xim_ic) XmbResetIC(fl_xim_ic);
 }
 
-int Fl_X11_Screen_Driver::text_display_can_leak() {
+int Fl_X11_Screen_Driver::text_display_can_leak() const {
 #if USE_XFT
   return 1;
 #else