Create primary monitor when using wxDisplay default ctor

Previously, the first monitor was created instead and while it was often
also the primary one, this wasn't always the case.

In particular, this makes wxGetDisplayPPI() always return something
reasonable instead of returning (0, 0) when the first monitor is not the
primary one, as expected by plenty of code, including our own printing
sample, which divides by the values returned from wxGetDisplayPPI()
without checking if they're zero.

include/wx/private/display.h

@@ -37,6 +37,9 @@ public:
         return m_impls[n];
     }
 
+    // Return the primary display object, creating it if necessary.
+    wxDisplayImpl* GetPrimaryDisplay();
+
     // get the total number of displays
     virtual unsigned GetCount() = 0;