Read characters from the virtual display
unsigned int SLsmg_read_raw (unsigned short *buf, unsigned int len)
SLsmg_read_raw attempts to read len characters from the
current position on the virtual display into the buffer specified by
buf. It returns the number of characters actually read. This
number will be less than len if an attempt is made to read
past the right margin of the display.
The purpose of the pair of functions, SLsmg_read_raw and
SLsmg_write_raw, is to permit one to copy the contents of one
region of the virtual display to another region.
SLsmg_char_at, SLsmg_write_raw