Add set_cursor_pos_unchecked method.

[cr1901]

In light of set_cursor_pos working exclusively in terms of screen coordinates, I needed a function that let me set offscreen addresses to values to work with scrolling.

[ColinTimBarndt]

#54 would solve this problem by allowing these off-screen coordinates (and still checking them).

[cr1901]

If it means that I can stop using my own fork of the driver, I'm not against learning your API changes (though your changes seem like they'd take longer to be reviewed :P).