Return display PPI from wxDC::GetPPI() in wxGTK3

For wxDC associated with a window, such as wx{Window,Client,Paint}DC,
this method should return the window PPI, not the hard coded 72 DPI of
Cairo graphics context.

This still doesn't work correctly when using multiple monitors with
different DPI settings, but is still a (modest) improvement.

src/gtk/dc.cpp

@@ -15,6 +15,7 @@
 #include "wx/dcclient.h"
 #include "wx/dcmemory.h"
 #include "wx/dcscreen.h"
+#include "wx/gdicmn.h"
 #include "wx/icon.h"
 #include "wx/gtk/dc.h"
 
@@ -237,6 +238,18 @@ void* wxGTKCairoDCImpl::GetCairoContext() const
     return cr;
 }
 
+wxSize wxGTKCairoDCImpl::GetPPI() const
+{
+    if ( m_window )
+    {
+        return wxGetDisplayPPI();
+    }
+
+    // For a non-window-based DC the concept of PPI doesn't make much sense
+    // anyhow, so just return the hardcoded value used by the base class.
+    return wxGCDCImpl::GetPPI();
+}
+
 //-----------------------------------------------------------------------------
 
 wxWindowDCImpl::wxWindowDCImpl(wxWindowDC* owner, wxWindow* window)