The layers library supports three types of layers: simple refresh, smart
refresh and super bitmap.  The type of the layer, specified by the Flags
field in the layers structure, determines what facilities the layer
provides.

    Use Only One Layer Type Flag
    ----------------------------
    The three layer-type Flags are mutually exclusive.  That is, only one
    layer-type flag (LAYERSIMPLE, LAYERSMART and LAYERSUPER) should be
    specified.

 simple refresh layer    super bitmap layer 
 smart refresh layer     backdrop layer