fix warning

Graphics/drawContext.h

@@ -76,6 +76,8 @@ class drawTransformScaled : public drawTransform {
 // widget toolkit (FLTK, Qt, etc.)
 class drawContextGlobal {
  public:
+  drawContextGlobal(){}
+  virtual ~drawContextGlobal(){}
   virtual void draw(){}
   virtual void drawCurrentOpenglWindow(bool make_current){}
   virtual int getFontIndex(const char *fontname){ return 0; }