When Tk creates a new window it assigns it the default visual
characteristics (visual, depth, and colormap) for its screen.
Tk_SetWindowVisual may be called to change them.
Tk_SetWindowVisual must be called before the window has
actually been created in X (e.g. before Tk_MapWindow or Tk_MakeWindowExist has been
invoked for the window). The safest thing is to call
Tk_SetWindowVisual immediately after calling Tk_CreateWindow. If tkwin
has already been created before Tk_SetWindowVisual is called
then it returns 0 and does not make any changes; otherwise it
returns 1 to signify that the operation completed successfully.
Note: Tk_SetWindowVisual should not be called if you just
want to change a window's colormap without changing its visual or
depth; call Tk_SetWindowColormap instead.