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/window.h

@@ -92,6 +92,7 @@ public:
 
     virtual int GetCharHeight() const wxOVERRIDE;
     virtual int GetCharWidth() const wxOVERRIDE;
+    virtual double GetContentScaleFactor() const wxOVERRIDE;
 
     virtual void SetScrollbar( int orient, int pos, int thumbVisible,
                                int range, bool refresh = true ) wxOVERRIDE;