screen is 2d. treat center of screen as pivot. create vector from pivot to the light's location on screen. save length of the vector. use -'ve of unit vector and different mulitpliers on the length of the vector to find the centers of the flare pieces. render them in 2d on top of normal frame. done. lens flares in a straight line.
some call this approach a hack. try it. If it's enough, use it.