A super bitmap layer is similar to a smart refresh layer.  it too has a
back-up area for rendering graphics for currently obscured parts of the
display.  Whenever an obscured area is made visible, the corresponding
part of the backup area is copied to the display automatically.

However, it differs from smart refresh in that:

  * The back-up bitmap is user-supplied, rather than being allocated
    dynamically by the system.

  * The back-up bitmap may be as large or larger than the the current
    size of the layer. It may also be larger than the maximum size of the
    layer.

To see a larger portion of a super bitmap on-display, use sizelayer().  to
see a different portion of the super bitmap in the layer, use
scrolllayer().

When the graphics routines perform drawing commands, part of the drawing
appears in the common bitmap (the on-display portion). any drawing outside
the displayed portion itself is rendered into the super bitmap. When the
layer is scrolled or sized, the layer contents are copied into the super
bitmap, the scroll or size positioning is modified, and the appropriate
portions are then copied back into the layer. (Refer to the graphics
library functions syncsbitmap() and copysbitmap().