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.

interface/wx/display.h

@@ -16,6 +16,12 @@
 class wxDisplay
 {
 public:
+    /**
+        Default constructor creating wxDisplay object representing the primary
+        display.
+     */
+    wxDisplay();
+
     /**
         Constructor, setting up a wxDisplay instance with the specified
         display.
@@ -24,7 +30,7 @@ public:
             The index of the display to use. This must be non-negative and
             lower than the value returned by GetCount().
     */
-    wxDisplay(unsigned int index = 0);
+    explicit wxDisplay(unsigned int index);
 
     /**
         Constructor creating the display object associated with the given