Fix handling of second auxiliary mouse button events in wxMSW

The test for AUX2 mouse button was wrong and checked whether the button
was pressed, instead of checking whether the event was generated by it.

Check the event source correctly by comparing wParam with XBUTTON2 and
not MK_XBUTTON2.

Closes https://github.com/wxWidgets/wxWidgets/pull/753

src/msw/window.cpp

@@ -5547,7 +5547,7 @@ bool wxWindowMSW::HandleMouseEvent(WXUINT msg, int x, int y, WXUINT flags)
         case WM_XBUTTONDOWN:
         case WM_XBUTTONUP:
         case WM_XBUTTONDBLCLK:
-            if ( flags & MK_XBUTTON2 )
+            if (HIWORD(flags) == XBUTTON2)
                 msg += wxEVT_AUX2_DOWN - wxEVT_AUX1_DOWN;
     }