"Fixed" manual WinDOS window positioning

 When moving the window with the mouse, GetClientRect suddenly
 returned nonsense and sgui_window_move launched the window into
 lunar orbit. Now we don't adjust the position anymore and the
 move window function does not move the client rect but the entire
 window instead (boo hoo).
Nullified back buffer pointer in sgui_window_internal_deinit

  - When destroying a window, the sgui_window_internal_deinit function
    is called, which destroys the canvas, but (up to now) does not
    nullify it.
  - On WinDOS, a WM_DESTROY message is sent
  - The message function tries to redraw the canvas (if required)
  - The canvas pointer is not NULL and points into nowhere
  - Result: endless debugging and pointer chasing sessions