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