Adding spciial case for MacOS which has a base DPI of 72.

See issue #3547

rtgui/rtscalable.h

@@ -38,6 +38,15 @@ protected:
 
 
 public:
+
+#ifdef __APPLE__
+    static constexpr double baseDPI = 72;
+    static constexpr double baseHiDPI = 144;
+#else
+    static constexpr double baseDPI = 96;
+    static constexpr double baseHiDPI = 192;
+#endif
+
     static void init(Gtk::Window *window);
     static void cleanup();
     static double getDPI ();