This example shows how to create a dual-playfield display.  Note that this
technique is only valid for screen modes which support dual-playfield, do
not try to convert other modes.

Setting up dual playfield mode in the openscreen() call is not the best
method of obtaining a dual playfield viewport for a screen.  It is better
to open a standard screen, passing to Intuition (or letting Intuition
create) only one of the playfield bitmaps (the front one).  Next allocate
and set up a second bitmap, its bitplanes, and a rasinfo structure
installing these into the new screen's viewport.  Update the viewport
modes to include DUALPF and call makescreen() and rethinkdisplay().  this
method, shown in the example below, keeps Intuition rendering (gadgets,
menus, windows) in a single playfield.

     dualplayfield.c