(svn r2448) General cleanup of rail related code, more to follow.
* Add: rail.[ch] for rail-related enums and wrapper functions.
* Codechange: Removed dozens of magic numbers with below enums.
* Codechange: Rewrote CheckTrackCombination().
* Add: TILE_SIZE, TILE_PIXELS and TILE_HEIGHT constants.
* Add: enums RailTileType, RailTileSubtype, SignalType to mask against the map arrays.
* Add: enums Track, TrackBits, Trackdir, TrackdirBits for railway track data. (Note that the old RAIL_BIT constants are replaced by TRACK_BIT ones).
* Add: enums Direction and DiagDirection
* Codechange: Moved a bunch of track(dir) related lookup arrays from npf.[ch] to rail.[ch].
* Codechange: move RailType enum from tile.h to rail.h.
* Add: Wrapper functions for masking signal status in the map arrays: SignalAlongTrackdir, SignalAgainstTrackdir and SignalOnTrack.
* Add: Wrapper functions to access rail tiles, using above enums
* Add: Wrapper functions to modify tracks, trackdirs, directions, etc.
* Add: Wrapper functions for all lookup arrays in rail.[ch] (Arrays are still used in parts of the code)
* Codechange: Renamed some variables and arguments to better represent what they contain (railbit -> track, bits -> trackdirbits, etc.).
* Codechange: Don't use FindLandscapeHeight() in CmdRemoveSingleRail(), since it returns way too much info. Use GetTileSlope() instead.
* Codechange: [NPF] Removed some unused globals and code from npf.c.

#ifndef NEWGRF_H
#define NEWGRF_H

#include "sprite.h"
#include "station.h"

typedef struct GRFFile GRFFile;
struct GRFFile {
	char *filename;
	uint32 grfid;
	uint16 flags;
	uint16 sprite_offset;
	GRFFile *next;

	/* A sprite group contains all sprites of a given vehicle (or multiple
	 * vehicles) when carrying given cargo. It consists of several sprite
	 * sets.  Group ids are refered as "cargo id"s by TTDPatch
	 * documentation, contributing to the global confusion.
	 *
	 * A sprite set contains all sprites of a given vehicle carrying given
	 * cargo at a given *stage* - that is usually its load stage. Ie. you
	 * can have a spriteset for an empty wagon, wagon full of coal,
	 * half-filled wagon etc.  Each spriteset contains eight sprites (one
	 * per direction) or four sprites if the vehicle is symmetric. */

	int spriteset_start;
	int spriteset_numsets;
	int spriteset_numents;
	int spriteset_feature;

	int spritegroups_count;
	SpriteGroup *spritegroups;

	StationSpec stations[256];
};

extern int _grffile_count;
extern GRFFile *_first_grffile;

void InitNewGRFFile(const char *filename, int sprite_offset);
void DecodeSpecialSprite(const char *filename, int num, int spriteid, int stage);


#endif /* NEWGRF_H */