NAME
        CMD_WRITE -- write to a clip on the clipboard.

   FUNCTION
        This command writes data to the clipboard.  This data can be
        provided sequentially by clearing io_Offset for the initial
        write, and using the incremented value unaltered for
        subsequent writes.  If io_Offset is ever beyond the current
        clip size, the clip is padded with zeros.

        If this write is in response to a satisfymsg for a pending
        post, then the io_ClipID returned by the cbd_post command must
        be used.  Otherwise, a new ID is obtained by clearing the
        io_ClipID for the first write.  Subsequent writes must not
        alter the io_ClipID.

   IO REQUEST
        io_Message      mn_ReplyPort set up
        io_Device       preset by opendevice
        io_Unit         preset by opendevice
        io_Command      CMD_WRITE
        io_Length       number of bytes from io_Data to write
        io_Data         pointer to block of data to write
        io_Offset       usually zero if this is the initial write
        io_ClipID       zero if this is the initial write, ClipID of
                        the Post if this is to satisfy a post

   RESULTS
        io_Error        non-zero if an error occurred
        io_Actual       filled with the actual number of bytes written
        io_Offset       updated to next write position
        io_ClipID       the clip ID assigned to this write: do not
                        alter for subsequent writes