(svn r5363) Revert 5312, 5288, 5248, 3992, 3249, 3228
They were all aimed at fixing the terraform-into-tunnel problem, but introduced new sideeffects while doing so

(svn r5221) Make the assertion in SetTileType() more strict: "lower edge of map <=> VOID" instead of just "lower edge of map => VOID"
This works since the hack abusing SetTileType() in openttd.c is gone (r5171)

(svn r3992) -Fix: Rewrote the code to determine whether a rail-tile can be terraformed.
Fixes a bug where you could terraform a tunnel (fixed by r3228, but reverted that one)
Fixes a bug introduced by r3228 which allowed steep rail tiles resulting in ... unwanted effects such as display artifacts.
That means the terraform feature should not work as intended; it also uses _valid_tileh_slopes to determine valid configurations instead of hand-brewn stuff.
TODO: _terraform_err_tile and similar TileIndices should have INVALID_TILE as "unused", not 0. (0 is a valid tile).

(svn r2962) - const correctness for all Get* functions and most Draw* functions that don't change their pointer parameters
- change a lot of byte player types to PlayerID
- beautify header files, same "#endif /* filename */" ending

(svn r2635) Fix: [ntp/misc] Improve the old pathfinder. Changed it to A* instead of Dijkstra.
- Benchmark shows that NTP is now around 10x faster than NPF.
- Made IsTunnelTile macro to determine if a tile is a tunnel.
- Added some useful debugging functions for making tiles red / getting accurate timestamps.
- Remove old depot finding algorithm.
- Disable warning for signed/unsigned comparisons.

(svn r2595) -Codechange: Introduced "IsSteepTileh" to find whether a tile is steep
(i.e. spans two height levels) and use it throughout the code.
-Codechange: Add CanBuildDepotByTileh to find if a tile is suitable to
build a depot on it. Wraps some bitmagic which seems quite unreadable at
first glance

(svn r2473) - Add: VehicleMayEnterTile(), which checks if the tile owner of a tile is correct for a vehicle to enter it. Based upon glx's code.
- Fix: [ 1203769 ] [NPF] NPF tries to plan over bridges, through tunnels, over level crossings of other players. (glx)
- Codechange: Renamed TRANSPORT_MAX to TRANSPORT_END and added INVALID_TRANSPORT.
- Codechange: Moved IsLevelCrossing() from tile.h to rail.h
- Add: GetCrossingTransportType(), which returns the transport type (road, rail) of both tracks on a level crossing.
- Removed old TODO that was fulfilled already.

(svn r2450) * Codechange: Replaced all uses of the arrays in tile.h with calls to the associated wrapper functions.
* Codechange: Made npf.c use some map array accessing wrappers instead of direct access.
* Codechange/Fix: Named every enum in tile.h. Fixes a nasty bug on MSVC where arrays would be initialised with zeroes (tnx Asterix_)
* Removed magic numbers from tables in tile.c.
* Added some explicit casts in tile.h.

(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.

(svn r2358) Add macros for getting (GB) and setting (SB) a range of bits
Use them exemplarily to prettify (Get|Set)Tile(Type|Height)

(svn r1837) GetTileOwner returns Owner, not bool
Also assert() that the TileIndex is valid

(svn r1834) - Fix: NPF does not check the owner of its target, busses try to enter other players' depots. TODO
- Add: asserts to find the v->u.rail.track == 0 problem.
- Add: IsValidDepot(), IsValidTown(), IsValidSign(), IsValidVehicle(), IsValidStation()
- Add: GetTileOwner(), IsTileOwner()
- Codechange: Replaced IsShipDepotTile(), IsTrainDepotTile(), IsRoadDepotTile() by IsTileDepotType().
- Codechange: typedeffed the MAP_OWNERS as Owner. Should be used as variable type.
- Codechange: Replaced a few uint by TileIndex.

(svn r1718) Use the enum TileType as parameter/return type for [GS]etTileType() instead of plain int.
This makes it necessary to rename TileType() to GetTileType() because a type and a function may not share the same name.