SCRSAVE ( nTop, nLeft, nBottom, nRight) -> cScreen

   Saves the passed ScreenPart (Coordinates) in a variable.
   The Coordinates are stored in the header of the variable so
   that you don't have to pass them, when you restore the
   screen by calling ScrRest( cScreen )

   ( I could never remember them when using SaveScreen/RestScreen )

   The saved area is packed with the RLE-Procedure:

        Byte 0) Left   1)Top   2)Rows    3)Columns
             4)Repeats 5) Byte 6)Repeats 7) Byte ...........

   This decreases the amount of memory needed for storing screens
   dramatically.


   PARAMETERS:
   Coordinates, again

   RETURN VALUE:
   packed screen