Allow drawing HiDPI bitmaps with GTK3

1. Override wxWindow::GetContentScaleFactor() to use gdk_window_get_scale_factor()
   when available, and to use correct scale (1.0) otherwise, as wxDC::GetPPI()
   (used by overridden method) is not properly implemented for wxGTK
2. Record scale in wxBitmap(wxImage&) ctor and wxBitmap::CreateScaled()
3. Adjust cairo scale for drawing bitmap, and (inversely) for drawing on bitmap

include/wx/gtk/bitmap.h

@@ -90,7 +90,10 @@ public:
         { return Create(sz.GetWidth(), sz.GetHeight(), depth); }
     bool Create(int width, int height, const wxDC& WXUNUSED(dc))
         { return Create(width,height); }
-    
+#ifdef __WXGTK3__
+    virtual bool CreateScaled(int w, int h, int depth, double scale) wxOVERRIDE;
+    virtual double GetScaleFactor() const wxOVERRIDE;
+#endif
 
     virtual int GetHeight() const wxOVERRIDE;
     virtual int GetWidth() const wxOVERRIDE;