Corrected bug #1437. Corrected the cast of two short values from an lParam. lParam...

Corrected bug #1437. Corrected the cast of two short values from an lParam. lParam contains two signed-short, the position can be negative when another mouse event happens with a mouse_move (mouse_click, etc.)
The Microsoft documentation specifies NOT to use LOWORD and HIWORD macros to extract the x/y positions as it won't work correctly with multiple monitors. See http://msdn.microsoft.com/en-us/library/windows/desktop/ms645607(v=vs.85).aspx.