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.

include/wx/gtk/dc.h

@@ -32,9 +32,14 @@ public:
     virtual bool DoStretchBlit(int xdest, int ydest, int dstWidth, int dstHeight, wxDC* source, int xsrc, int ysrc, int srcWidth, int srcHeight, wxRasterOperationMode rop, bool useMask, int xsrcMask, int ysrcMask) wxOVERRIDE;
     virtual void* GetCairoContext() const wxOVERRIDE;
 
+    virtual wxSize GetPPI() const wxOVERRIDE;
+
 protected:
     int m_width, m_height;
 
+private:
+    bool TryGetWindowSize(wxSize& size, wxSize& sizeMM) const;
+
     wxDECLARE_NO_COPY_CLASS(wxGTKCairoDCImpl);
 };
 //-----------------------------------------------------------------------------