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.

src/common/dpycmn.cpp

@@ -77,6 +77,11 @@ wxIMPLEMENT_DYNAMIC_CLASS(wxDisplayModule, wxModule);
 // ctor/dtor
 // ----------------------------------------------------------------------------
 
+wxDisplay::wxDisplay()
+{
+    m_impl = Factory().GetPrimaryDisplay();
+}
+
 wxDisplay::wxDisplay(unsigned n)
 {
     wxASSERT_MSG( n == 0 || n < GetCount(),
@@ -89,7 +94,8 @@ wxDisplay::wxDisplay(const wxWindow* window)
 {
     const int n = GetFromWindow(window);
 
-    m_impl = Factory().GetDisplay(n != wxNOT_FOUND ? n : 0);
+    m_impl = n != wxNOT_FOUND ? Factory().GetDisplay(n)
+                              : Factory().GetPrimaryDisplay();
 }
 
 // ----------------------------------------------------------------------------
@@ -245,6 +251,24 @@ void wxDisplayFactory::ClearImpls()
     m_impls.clear();
 }
 
+wxDisplayImpl* wxDisplayFactory::GetPrimaryDisplay()
+{
+    // Just use dumb linear search -- there seems to be the most reliable way
+    // to do this in general. In particular, primary monitor is not guaranteed
+    // to be the first one and it's not obvious if it always contains (0, 0).
+    const unsigned count = GetCount();
+    for ( unsigned n = 0; n < count; ++n )
+    {
+        wxDisplayImpl* const d = GetDisplay(n);
+        if ( d && d->IsPrimary() )
+            return d;
+    }
+
+    // This is not supposed to happen, but what else can we do if it
+    // somehow does?
+    return NULL;
+}
+
 int wxDisplayFactory::GetFromWindow(const wxWindow *window)
 {
     wxCHECK_MSG( window, wxNOT_FOUND, "window can't be NULL" );