(svn r7718) -Fix (runknown): When pathfinding onto a bridge or tunnel end from
previous tile (but not warping from the opposite end) check the
enter direction. This fixes signal setting if a rail ends on the top of
a tunnel end.

(svn r7717) -Fix (runknown): When following path for signals, don't skip back to the
previous tile, as for tunnels & bridge ends the entering direction is wrong.

(svn r7620) -Fix: [OPF] signal update was incorrectly propagated:
- through incompatible rail types
- from under bridge track to the bridge ramp (peter1138)
- same for tunnels (from track on top of tunnel entry to the tunnel)

(svn r7565) -Codechange: Rework DEBUG functionality. Look for appropiate debugging levels to
use in debug.h. grfmsg() is now used as a specific debug-function for grf.

(svn r6406) -Codechange: Rename TileOffsByDir to TileOffsByDiagDir because it accepts
DiagDirections, and add TileOffsByDir that handles Directions.
-Codechange: Make the treeloop use TileOffsByDir().

(svn r5396) - Remove two fixed parameters
- Simplify an expresssion: c ? f(x) : f(y) -> f(c ? x : y)

(svn r5070) Merged the bridge branch
-Feature: Bridges can now be placed above:
Any railway track combination (excluding depots and waypoints)
Any road combination (excluding depots)
Clear tiles (duh), including fields
Tunnel entrances
Bridge heads

Thanks to Tron for idea and implementation, KUDr for the yapf synchronization and many others for hours of testing

There are still a number of visual problems remaining, especially when electric railways are on or under the bridge.
DO NOT REPORT THOSE BUGS FOR THE TIME BEING please.

(svn r3329) - Doc: Some documentation cleanups.
- Add: TracksOverlap() (from the map branch), TrackdirBitsToTrackBits(), DiagdirReachesTrackdirs(), DiagdirReachesTracks().
- Fix: Infinite loop in the pathfinder introduces in r3321.

(svn r3321) - Fix: A wrong use of the map m5 bits, where a previously calculated "bits" variable should have been used. This resulted in the pathfinder imagining junctions, which negatively affects performance somewhat (Darkvater).
- Fix: [ 1346377 ] Limiting the "depth" of the search tree fixes this assert.
Though the above fix seems to fix this bug too, it will only make it less likely to occur. The problem here was the StackedItem::depth field overflowing, which made the pathfinder think it was at the first tile again. Adding an explicit overflow check should fix this.

(svn r3077) static, const, bracing, indentation, 0 -> '\0'/NULL, typos in comments, excess empty lines, minor other changes
nothing spectacular, just some stuff, which piled up

(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 r2553) - Fix: [pathfinding] Remove old-old train pathfinder. Enhanced old pathfinder.
- Penalties for red signals and for slopes.
- Increased the search depth to work better with large train networks.

(svn r2514) - Codechange: [NPF] Move the checking of railtype into a funciton IsCompatibleRail().
- Codechange: [NPF] Check the railtype along a route against the engine type instead of against the previouse tile. This clears the way for electriefied rails.
- Add: [NPF] [ 1209644 ] A penalty for crossings (peter1138)

(svn r1386) Move TileIndexDiff to map.h
Move _tileoffs_by_dir to map.[ch] and encapsulate it in TileOffsByDir()

(svn r1208) -Fix: the owner-check introduced in r1203 now also works correctly for
bridges (the middle-parts don't have any owner)

(svn r1203) -Fix: the pathfinder no longer sees rail with an other owner as a
possible route. This also makes sure signals don't show red because
there is a train on the track of an other owner.

(svn r907) Sprinkle holy ANSI water:
- "inline" must before the return type (and after "static")
- Initialise all struct members, not just some of them
- Remove (one) spurious semicolon

(svn r607) -Patch: [ 985102 ] static cleanup
Thanks to lvoge

(svn r242) -Fix: Pathfinding bug for road vehicles introduced in r160 fixed (blathijs)
-Fix: Pathfinding under bridges bug for ships introduced in r160 fixed (Darkvater)

(svn r160) -Codechange: made GetTileTrackStatus more readable (blathijs)
-Fix: some minor fixes around GetTileTrackStatus (blathijs)