(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.

rail.c

@@ -0,0 +1,93 @@
+#include "rail.h"
+
+/* Maps a trackdir to the bit that stores its status in the map arrays, in the
+ * direction along with the trackdir */
+const byte _signal_along_trackdir[] = {
+	0x80, 0x80, 0x80, 0x20, 0x40, 0x10, 0, 0,
+	0x40, 0x40, 0x40, 0x10, 0x80, 0x20
+};
+
+/* Maps a trackdir to the bit that stores its status in the map arrays, in the
+ * direction against the trackdir */
+const byte _signal_against_trackdir[] = {
+	0x40, 0x40, 0x40, 0x10, 0x80, 0x20, 0, 0,
+	0x80, 0x80, 0x80, 0x20, 0x40, 0x10
+};
+
+/* Maps a Track to the bits that store the status of the two signals that can
+ * be present on the given track */
+const byte _signal_on_track[] = {
+	0xC0, 0xC0, 0xC0, 0x30, 0xC0, 0x30
+};
+
+/* Maps a diagonal direction to the all trackdirs that are connected to any
+ * track entering in this direction (including those making 90 degree turns)
+ */
+const TrackdirBits _exitdir_reaches_trackdirs[] = {
+	TRACKDIR_BIT_DIAG1_NE|TRACKDIR_BIT_LOWER_E|TRACKDIR_BIT_LEFT_N,  /* DIAGDIR_NE */
+	TRACKDIR_BIT_DIAG2_SE|TRACKDIR_BIT_LEFT_S |TRACKDIR_BIT_UPPER_E, /* DIAGDIR_SE */
+	TRACKDIR_BIT_DIAG1_SW|TRACKDIR_BIT_UPPER_W|TRACKDIR_BIT_RIGHT_S, /* DIAGDIR_SW */
+	TRACKDIR_BIT_DIAG2_NW|TRACKDIR_BIT_RIGHT_N|TRACKDIR_BIT_LOWER_W  /* DIAGDIR_NW */
+};
+
+/* TODO: Remove magic numbers from tables below just like
+ * _exitdir_reaches_trackdirs[] */
+
+const Trackdir _next_trackdir[14] = {
+	0,  1,  3,  2,  5,  4, 0, 0,
+	8,  9,  11, 10, 13, 12
+};
+
+/* Maps a trackdir to all trackdirs that make 90 deg turns with it. */
+const TrackdirBits _trackdir_crosses_trackdirs[] = {
+	0x0202, 0x0101, 0x3030, 0x3030, 0x0C0C, 0x0C0C, 0, 0,
+	0x0202, 0x0101, 0x3030, 0x3030, 0x0C0C, 0x0C0C
+};
+
+/* Maps a track to all tracks that make 90 deg turns with it. */
+const TrackBits _track_crosses_tracks[] = {
+	0x2, /* Track 1 -> Track 2 */
+	0x1, /* Track 2 -> Track 1 */
+	0x30, /* Upper -> Left | Right */
+	0x30, /* Lower -> Left | Right */
+	0x0C, /* Left -> Upper | Lower */
+	0x0C, /* Right -> Upper | Lower */
+};
+
+/* Maps a trackdir to the (4-way) direction the tile is exited when following
+ * that trackdir */
+const DiagDirection _trackdir_to_exitdir[] = {
+	0,1,0,1,2,1, 0,0,
+	2,3,3,2,3,0,
+};
+
+const Trackdir _track_exitdir_to_trackdir[][DIAGDIR_END] = {
+	{0,    0xff, 8,    0xff},
+	{0xff, 1,    0xff, 9},
+	{2,    0xff, 0xff, 10},
+	{0xff, 3,    11,   0xf},
+	{0xff, 0xff, 4,    12},
+	{13,   5,    0xff, 0xff}
+};
+
+const Trackdir _track_direction_to_trackdir[][DIR_END] = {
+	{0xff, 0,    0xff, 0xff, 0xff, 8,    0xff, 0xff},
+	{0xff, 0xff, 0xff, 1,    0xff, 0xff, 0xff, 9},
+	{0xff, 0xff, 2,    0xff, 0xff, 0xff, 10,   0xff},
+	{0xff, 0xff, 3,    0xff, 0xff, 0xff, 11,   0xff},
+	{12,   0xff, 0xff, 0xff, 4,    0xff, 0xff, 0xff},
+	{13,   0xff, 0xff, 0xff, 5,    0xff, 0xff, 0xff}
+};
+
+const Trackdir _dir_to_diag_trackdir[] = {
+	0, 1, 8, 9,
+};
+
+const DiagDirection _reverse_diagdir[] = {
+	2, 3, 0, 1
+};
+
+const Trackdir _reverse_trackdir[] = {
+	8, 9, 10, 11, 12, 13, 0xFF, 0xFF,
+	0, 1, 2,  3,  4,  5
+};