screentofront() and screentoback() make a screen either the frontmost or
the backmost screen.  If an application needs to render into a screen
before the screen becomes visible to the user, the screen may be opened
behind all other screens and later moved to the front when ready with
ScreenToFront().

    VOID ScreenToFront( struct Screen * )
    VOID ScreenToBack ( struct Screen * )

Depth control of screens is also available through the depth arrangement
gadget in the screen's title bar or through keyboard shortcuts.  The N key
with the Left-Amiga qualifier moves the Workbench screen to front.  The M
key with the Left-Amiga qualifier moves the frontmost screen to back.
Repeated selection of Left-Amiga-M will cycle through available screens.
These keys are processed through the keymap and will retain their value
even if the key location changes.