NAME
        CMD_READ -- read from a clip on the clipboard.

   FUNCTION
        The read function serves two purposes.

        When io_Offset is within the clip, this acts as a normal read
        request, and io_Data is filled with data from the clipboard.
        The first read request should have a zero io_ClipID, which
        will be filled with the ID assigned for this read.  Normal
        sequential access from the beginning of the clip is achieved
        by setting io_Offset to zero for the first read, then leaving
        it untouched for subsequent reads.  If io_Data is null, then
        io_Offset is incremented by io_Actual as if io_Length bytes
        had been read: this is useful to skip to the end of file
        by using a huge io_Length.

        When io_Offset is beyond the end of the clip, this acts as a
        signal to the clipboard device that the application is
        through reading this clip.  Realize that while an application
        is in the middle of reading a clip, any attempts to write new
        data to the clipboard are held off.  This read past the end
        of file indicates that those operations may now be initiated.

   IO REQUEST
        io_Message      mn_ReplyPort set up
        io_Device       preset by opendevice
        io_Unit         preset by opendevice
        io_Command      CMD_READ
        io_Length       number of bytes to put in data buffer
        io_Data         pointer to buffer of data to fill, or null to
                        skip over data
        io_Offset       byte offset of data to read
        io_ClipID       zero if this is the initial read

   RESULTS
        io_Error        non-zero if an error occurred
        io_Actual       filled with the actual number of bytes read
        io_Data         (the buffer now has io_Actual bytes of data)
        io_Offset       updated to next read position, which is
                        beyond EOF if io_Actual != io_Length
        io_ClipID       the clip ID assigned to this read: do not
                        alter for subsequent reads