FindSpot now checks the whole spot for valid spawning positions

Previously FindSpot only checked the downstream end if it was drivable.

Now, if it was not a valid spawning position, it reduces the length of the spot by what would've been consumed by a spawn, and checks again and again, until it either finds a valid position or reaches the end of the spot.

This makes lanes with 2 or more sections where the last section is not drivable (like a highway entry lane where the start of it is drivable but the end is a shoulder), be able to spawn in the drivable sections.