(svn r10280) [0.5] -Backport from trunk (r10109, r10219, r10222, r10230, r10246, r10258):

- Fix: Do not look in every direction for tunnels when building one, one direction is enough (r10258)
- Fix: Take the age of the front vehicle for station rating (r10246)
- Fix: Terraforming wipes out canals. Now you always have to remove the canal before terraforming, instead of "just" removing the canal [FS#594] (r10240)
- Fix: Only 2 trains could crash at one time as collision checking stopped on the first hit. This could technically cause desyncs in network games as the collision hash order is not guaranteed [FS#892] (r10222)
- Fix: Land under foundations was terraform when it shouldn't be terraformed [FS#882, FS#890] (r10219)
- Fix: Some NewGRFs use the same (unused in the "current" climate) sprite IDs. Normally this gives some artefacts, but when one NewGRF expects it to be a sprite and another NewGRF overwrites it with a non-sprite nasty things happen (drawing a non-sprite crashes OTTD) [FS#838] (r10109)

clear_cmd.c

@@ -17,6 +17,7 @@
 #include "unmovable_map.h"
 #include "genworld.h"
 #include "industry.h"
+#include "water_map.h"
 
 typedef struct TerraformerHeightMod {
 	TileIndex tile;
@@ -111,7 +112,7 @@ static int TerraformProc(TerraformerState *ts, TileIndex tile, int mode)
 		// basement and then you raise/lower the other corner.
 		tileh = GetTileSlope(tile, &z);
 		if (tileh == unsafe_slope[mode] ||
-				tileh == ComplementSlope(unsafe_slope[mode])) {
+				tileh == (SLOPE_STEEP | ComplementSlope(unsafe_slope[mode]))) {
 			_terraform_err_tile = tile;
 			_error_message = STR_1008_MUST_REMOVE_RAILROAD_TRACK;
 			return -1;
@@ -136,6 +137,13 @@ static int TerraformProc(TerraformerState *ts, TileIndex tile, int mode)
 		}
 	}
 
+	/* Canals can't be terraformed */
+	if (IsClearWaterTile(tile) && IsCanal(tile)) {
+		_terraform_err_tile = tile;
+		_error_message = STR_MUST_DEMOLISH_CANAL_FIRST;
+		return -1;
+	}
+
 	ret = DoCommand(tile, 0,0, ts->flags & ~DC_EXEC, CMD_LANDSCAPE_CLEAR);
 
 	if (CmdFailed(ret)) {