new segment request: chicanes

A chicane is not a segment in itself. What's needed is the ability to add curves to the props (lanes, sidewalks and markings). What I would suggest is to have the ability to add a marker to the prop (or all props in that alignment) which allows us to shift it sideways (adding a gentle curve to the transition). One extra option related to this would be to add the ability to these markers to become extremities thus terminating the prop. This would allow two things to happen : 1 - When shifting a lane sideways we wouldn't need to cover it (ie no need to cover a parking lane with a sidewalk) or change it's Z - scale 2 - We would sometimes use to only make a prop shorter again without changing the Z-scale (ie a sidewalk that is used as a pedestrian island)

3DStreet / 3dstreet

new segment request: chicanes #376