Add necessary virtual qualifier to ~Fl_Font_Descriptor() and derived.

FL/Fl_Graphics_Driver.H

@@ -383,7 +383,7 @@ public:
   Fl_Font_Descriptor *next;
   Fl_Fontsize size; /**< font size */
   Fl_Font_Descriptor(const char* fontname, Fl_Fontsize size);
-  FL_EXPORT ~Fl_Font_Descriptor() {}
+  virtual FL_EXPORT ~Fl_Font_Descriptor() {}
   short ascent, descent;
   unsigned int listbase;// base of display list, 0 = none
 };

src/drivers/Cairo/Fl_Cairo_Graphics_Driver.H

@@ -32,7 +32,7 @@ typedef struct _PangoFontDescription PangoFontDescription;
 class Fl_Cairo_Font_Descriptor : public Fl_Font_Descriptor {
 public:
   Fl_Cairo_Font_Descriptor(const char* fontname, Fl_Fontsize size, PangoContext *context);
-  FL_EXPORT ~Fl_Cairo_Font_Descriptor();
+  virtual FL_EXPORT ~Fl_Cairo_Font_Descriptor();
   PangoFontDescription *fontref;
   int **width; // array of arrays of character widths
   int line_height;

src/drivers/GDI/Fl_Font.H

@@ -34,7 +34,7 @@ public:
 #  if HAVE_GL
   char glok[64];
 #  endif // HAVE_GL
-  FL_EXPORT ~Fl_GDI_Font_Descriptor();
+  virtual FL_EXPORT ~Fl_GDI_Font_Descriptor();
 };
 
 extern FL_EXPORT Fl_Fontdesc *fl_fonts; // the table

src/drivers/Quartz/Fl_Font.H

@@ -29,7 +29,7 @@
 class Fl_Quartz_Font_Descriptor : public Fl_Font_Descriptor {
 public:
   Fl_Quartz_Font_Descriptor(const char* fontname, Fl_Fontsize size);
-  FL_EXPORT ~Fl_Quartz_Font_Descriptor();
+  virtual FL_EXPORT ~Fl_Quartz_Font_Descriptor();
 # if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_5
   CTFontRef fontref;
   // the unicode span is divided in 512 blocks of 128 characters

src/drivers/Xlib/Fl_Font.H

@@ -49,7 +49,7 @@ public:
   char glok[64];
 #  endif // HAVE_GL
 #  endif // USE_XFT
-  FL_EXPORT ~Fl_Xlib_Font_Descriptor();
+  virtual FL_EXPORT ~Fl_Xlib_Font_Descriptor();
 };
 
 //extern FL_EXPORT Fl_Font_Descriptor *fl_fontsize; // the currently selected one