Add wxDisplay::GetPPI() to use instead of wxGetDisplayPPI()

While this is not done for all the ports yet, the new API allows
returning different PPI values for different monitors, unlike the old
(and still existing, but implemented in terms of the new one) global
function.

src/common/dpycmn.cpp

@@ -29,6 +29,8 @@
     #include "wx/module.h"
 #endif //WX_PRECOMP
 
+#include "wx/math.h"
+
 #include "wx/private/display.h"
 
 #if wxUSE_DISPLAY
@@ -122,6 +124,13 @@ wxRect wxDisplay::GetClientArea() const
     return m_impl->GetClientArea();
 }
 
+wxSize wxDisplay::GetPPI() const
+{
+    wxCHECK_MSG( IsOk(), wxSize(), wxT("invalid wxDisplay object") );
+
+    return m_impl->GetPPI();
+}
+
 int wxDisplay::GetDepth() const
 {
     wxCHECK_MSG( IsOk(), 0, wxT("invalid wxDisplay object") );
@@ -180,6 +189,26 @@ bool wxDisplay::ChangeMode(const wxVideoMode& mode)
     return *gs_factory;
 }
 
+// ============================================================================
+// wxDisplayImpl implementation
+// ============================================================================
+
+wxSize wxDisplayImpl::GetPPI() const
+{
+    const wxSize pixels = GetGeometry().GetSize();
+    const wxSize mm = GetSizeMM();
+
+    if ( !mm.x || !mm.y )
+    {
+        // Physical size is unknown, return a special value indicating that we
+        // can't compute the resolution -- what else can we do?
+        return wxSize(0, 0);
+    }
+
+    return wxSize(wxRound((pixels.x * inches2mm) / mm.x),
+                  wxRound((pixels.y * inches2mm) / mm.y));
+}
+
 // ============================================================================
 // wxDisplayFactory implementation
 // ============================================================================