Horizoff

Horizoff appears to be the auto 'look up/down' based on sector slope when not using mouse aiming. Horiz and horizoff are used together to calculate the view.

Horiz is initialised to 140 at level entry. The 'O' versions (ohoriz, ohorizoff) hold the values of the last game tic for render-time interpolation. (XXX: why is this here?)