Make GAL_DISPLAY_OPTIONS have virtual destructor

This is needed to allow proper deletion of derivative objects using this
as a base class.

include/gal/gal_display_options.h

@@ -88,6 +88,9 @@ namespace KIGFX
     public:
         GAL_DISPLAY_OPTIONS();
 
+        virtual ~GAL_DISPLAY_OPTIONS()
+        {}
+
         OPENGL_ANTIALIASING_MODE gl_antialiasing_mode;
 
         CAIRO_ANTIALIASING_MODE cairo_antialiasing_mode;