(svn r11011) -Fix [FS#1129]: GetFirstVehicleInChain did change the game state while being marked const.
-Codechange: do not brute force determine the first vehicle in the chain or previous vehicle, but do it by properly accounting the previous and first pointers when updating the next pointer. This gives a performance increase of about 15% when there are a lot of vehicles in the game.

/* $Id$ */

KNOWN BUGS / PROBLEMS:

bridges:
- Clearing tiles which may not have a bridge above can cause random bridge pieces to appear, which leads to crash in the long run

electrified rails:
Normal and elrail depots look the same. Use 'X' (transparent buildings)
   to distinguish between them
Missing curors / icons for construction (currently using the conventional ones)