Central optics cut out defined using the side lug positions for all of the corners, not just for the side corners

From #393 (closed) and !429 (merged):

central_optics_cut_out_projection(params) is the culprit. It is extruded into central_optics_cut_out(params) and used for objective_mount_body(params, h).

module central_optics_cut_out_projection(params) {
    // Central cut-out for optics of main body
    hull(){
        translate_y(back_lug_x_pos(params)+1.5-14/2){
            square([14,2*tiny()],center=true);
        }
        square([2*(back_lug_x_pos(params)-flex_dims().x),1],center=true);
        translate_y(8-(back_lug_x_pos(params)-flex_dims().x-tiny())){
            square([16,2*tiny()],center=true);
        }
    }
}

Only the middle, untranslated, square should depend on back_lug_x_pos. The other ones should be related to the positions of corners at the front and back of the inner wall.