(svn r7385) -Fix: FS#418 Deleting Train in depot with autoreplace failes
This turned out to be due to continue to drag the old vehicle, that autoreplace sold
This could also be triggered if more than one player used the same company
Now deleting a vehicle will remove all depot highlights of that vehicle

(svn r7331) - Codechange: Rename all memory pool macro's and types to "old pool", so the new pool implementation can be committed alongside it.
- Codechange: Rename pool.[ch] to oldpool.[ch].

(svn r6513) -Codechange: unified the code to draw depot windows
This change is intended to make it easier to make depot behaviour consistent
and faster to code when adding more features in the future
The user interface should hopefully not be affected by this

(svn r6381) -Cleanup: make the '/* */' comments that span multiple lines more uniform.
-Cleanup: whitespace alignment of a few tables.

(svn r6141) -Codechange: introduced DepotID and used it as much as possible
-Codechange: DeleteDepot removes a depot from the pool
-Codechange: DestroyDepot is called by DeleteDepot to remove all things where a depot depends on.
Last 2 changes to prepare for new pool system. Not pretty now, will be soon.
-Codechange: Removed DoDeleteDepot as it was stupid

(svn r6053) -Codechange: renamed all IsXXXIndex to IsValidXXXID
-Codechange: IsValidXXXID now also checks if XXX is really valid, not if the number is within range
Both changes again in preperation of the new mem-pool system, which requires this.
IsValidXXXID is not a bit less pretty, but that will be cleaned up after the new mem-pool system

(svn r6047) -Codechange: FOR_ALL now _only_ loops valid items, and skips invalid ones
-Codechange: use IsValidXXX where ever possible
Note: both changes to prepare for new pool system, which needs those changes.
For every pool there are 2 ugly lines, which will be removed when done
implementing new pool system.
Based on FS#13 by blathijs, partly implemented.

(svn r5934) -Cleanup: forgot some conversions to Year and to Date
-Cleanup: use _cur_year instead of _date for some (year based) comparisons
-Cleanup: remove a magic number in favour of another (less) magic number

(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 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 r2222) Check the parameters of Cmd{Insert,Delete,Modify,Skip}Order() and CmdRestoreOrderIndex():
- Check if the vehicle exists
- Check if the vehicle belongs to the correct player
- Check if the new order is valid (type, destination, flags) (CmdInsertOrder)

(svn r2154) - Fix: [NPF] Vehicles should no longer try to drive through the back of depots and road stations.
- Add: GetDepotDirection() wrapper function.
- Fix: [NPF] Ships can now actually reach buoys.

(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 r1817) -Codechange: Moved depot-functions to depot.c
-Codechange: Added wrappers around depot-access (GetDepot no exists)
-Codechange: Made depot-functions a bit more logic (no longer
GetDepotByTile crashes your game when you request it on a non-depot tile)
-Add: made depots dynamic (yes, 64k depots are possible now)