SetBkMode()      Set the specified background mode
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      SetBkMode( <hDC>, <nNewBkMode> )    -->  <nPrevBkMode>

 The SetBkMode function sets the specified background mode. The background mode defines
 whether the system removes existing background colors on the drawing surface
 before drawing text, hatched brushes, or any pen style that is not a solid
 line.

 <hDC>           Handle (numeric value) of a Device Context.

 <nNewBkMode>    Numeric value specifying one of the followings:

       OPAQUE   2   Background is filled with the current background color before
                    the text, hatched brush, or pen is drawn. This is the default
                    background mode.

  TRANSPARENT   1   Background is not changed before drawing.


 <nPrevBkMode>      Return value is the previous background mode, if the
                    function is successful.