fill_buf
static char *fill_buf(char *, size_t, size_t, size_t);
ptr = fill_buf(ptr, Selarea.sa_y1, Selarea.sa_x1,
ptr = fill_buf(ptr, Selarea.sa_y1, Selarea.sa_x1, l);
ptr = fill_buf(ptr, r, 0, row_length(r));
ptr = fill_buf(ptr, Selarea.sa_y2, 0, l);