Fix RTL mirroring for wxClientDC, etc with GTK3

The changes to the Cairo context must be done before wxGC::CreateFromNative()
is called, otherwise they will be overwritten

include/wx/gtk/dc.h

@@ -41,6 +41,7 @@ public:
 protected:
     // Set m_size from the given (valid) GdkWindow.
     void InitSize(GdkWindow* window);
+    void AdjustForRTL(cairo_t* cr);
 
     wxSize m_size;
     wxLayoutDirection m_layoutDir;