Modified wxGraphicsPath concatenation (with AddPath) for Direct2D renderer.

Because wxGraphicsPath comprises current ID2D1PathGeometry object and the collection of transformed (sub-)geometries (ID2D1TransformedGeometry objects) so to concatenate two paths we need to concatenate their current geometries as well as to combine the collections of transformed geometries.

interface/wx/graphics.h

@@ -98,6 +98,8 @@ public:
     /**
         Adds another path onto the current path. After this call the current
         point will be at the added path's current point.
+        For Direct2D the path being appended shouldn't contain
+        a started non-empty subpath when this function is called.
     */
     virtual void AddPath(const wxGraphicsPath& path);