Fixed closing sub-path of wxGraphicsPath with Direct2D renderer.

When sub-path is closed with CloseSubpath() then current point should be moved to the joined endpoint of the sub-path (what is equivalent of moving it to the starting point of the sub-path). To implement this behaviour we need to store on our own the starting point of the figure in a dedicate data member because native D2D renderer apparently doesn't offer any support for retrieving it.

See #17520

interface/wx/graphics.h

@@ -99,7 +99,8 @@ public:
                                      wxDouble x, wxDouble y);
 
     /**
-        Appends a rectangle as a new closed subpath.
+        Appends a rectangle as a new closed subpath. After this call
+        the current point will be at (@a x, @a y).
     */
     virtual void AddRectangle(wxDouble x, wxDouble y, wxDouble w, wxDouble h);
 
@@ -110,7 +111,8 @@ public:
                                      wxDouble h, wxDouble radius);
 
     /**
-        Closes the current sub-path.
+        Closes the current sub-path. After this call the current point will be
+        at the joined endpoint of the sub-path.
     */
     virtual void CloseSubpath();