Returns a block of image data containing the specifed rectangular area, in the specified component, as a reference to the internal buffer (see below). The rectangular area is specified by the coordinates and dimensions of the 'blk' object.

       <p>The area to return is specified by the 'ulx', 'uly', 'w' and 'h'
       members of the 'blk' argument. These members are not modified by this
       method.</p><p>The data returned by this method can be the data in the internal
       buffer of this object, if any, and thus can not be modified by the
       caller. The 'offset' and 'scanw' of the returned data can be
       arbitrary. See the 'DataBlk' class.</p><p>The returned data has its 'progressive' attribute unset
       (i.e. false).</p>