Primitive follow camera

Quickly put together the very basics of a follow camera. Very crude and only follows the lead carriage right now - but it will be good enough for a while.

Currently this is just based on breaking out of a fixed distance margin in the centre - need to migrate to a proper 'camera view box' instead.

But that's later on: I'm slowly checking off a bunch of minor errands that need to be done in order to push the game toward being playable on a very basic level. Rail paths and station interactions are the big ticket items planned for the weekend.