[cairo] Alternate cairo_path_t representation

[Paolo Bonzini]

> This is unfortunate because the data for both current_point and last_move_to
> is always available nearby.  The latter is because cairo always does an
> implicit move_to after close_path.  So, during LINE_TO, CURVE_TO, and
> CLOSE_PATH, current_point is always equal to data[-1], and during CLOSE_PATH,
> last_move_to is always equal to data[2].  But this is too ugly to advertise
> and to use.

Well, the current_point part is not so ugly to refer to and could be
documented.

Paolo