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/display.h

@@ -42,12 +42,14 @@ class WXDLLIMPEXP_FWD_CORE wxDisplayImpl;
 class WXDLLIMPEXP_CORE wxDisplay
 {
 public:
+    // default ctor creates the object corresponding to the primary display
+    wxDisplay();
+
     // initialize the object containing all information about the given
     // display
     //
-    // the displays are numbered from 0 to GetCount() - 1, 0 is always the
-    // primary display and the only one which is always supported
-    wxDisplay(unsigned n = 0);
+    // the displays are numbered from 0 to GetCount() - 1
+    explicit wxDisplay(unsigned n);
 
     // create display object corresponding to the display of the given window
     // or the default one if the window display couldn't be found