Fixed setting clipping box for wxDC (MSW).

Because CreateRectRgn() Win API requires that (x1,y1) parameters represent the top-left corner of the clipping box so if a box with negative values of the width or height is passed to wxMSWDCImpl::DoSetClippingRegion() (what means that (x,y) doesn't represent top-left corner) we need to recalculate passed parameters to get the box with (x,y) at top-left corner.

src/msw/dc.cpp

@@ -625,6 +625,19 @@ void wxMSWDCImpl::DoSetClippingRegion(wxCoord x, wxCoord y, wxCoord w, wxCoord h
     // manually
     //
     // FIXME: possible +/-1 error here, to check!
+    // (x,y) has to represent the left-top corner of the region
+    // so if size values are negative we need to recalculate
+    // parameters of the region to get (x,y) at this corner.
+    if ( w < 0 )
+    {
+        w = -w;
+        x -= (w - 1);
+    }
+    if ( h < 0 )
+    {
+        h = -h;
+        y -= (h - 1);
+    }
     HRGN hrgn = ::CreateRectRgn(LogicalToDeviceX(x),
                                 LogicalToDeviceY(y),
                                 LogicalToDeviceX(x + w),