add an override_redirect InputOnly window utility.

The intent was the following:
 This is mostly a copy of override.c, it lets you map a small window
 (1x1 at 0,0 by default) that openbox will treat like the root window,
 useful for scrolling to change desktops over fullscreen and maximized
 windows.

Since then openbox has learned to ignore other's override redirect windows
but I think it might still be useful to have this util.