Add static wxAnimationCtrl::CreateCompatibleAnimation()

This may be more convenient to use than CreateAnimation() if there is no
wxAnimationCtrl object at hand.

include/wx/gtk/animate.h

@@ -65,6 +65,8 @@ public:     // public API
 
     bool SetBackgroundColour( const wxColour &colour ) wxOVERRIDE;
 
+    static wxAnimation CreateCompatibleAnimation();
+
 protected:
     virtual wxAnimationImpl* DoCreateAnimationImpl() const wxOVERRIDE;