Codechange: pass "ground vehicle" to GetTileSlopeZ since for tunnel/bridges there are two states

Previously it checked the position in non-driving direction to "guess" whether
a ground vehicle was using the function, so on tunnels/bridges it could either
return the Z of the (virtual) ground compared to the Z of the path the vehicle
would take.

src/tile_cmd.h

@@ -73,7 +73,19 @@ struct TileDesc {
  * @param ti Information about the tile to draw
  */
 typedef void DrawTileProc(TileInfo *ti);
-typedef int GetSlopeZProc(TileIndex tile, uint x, uint y);
+
+/**
+ * Tile callback function signature for obtaining the world \c Z coordinate of a given
+ * point of a tile.
+ *
+ * @param tile The queries tile for the Z coordinate.
+ * @param x World X coordinate in tile "units".
+ * @param y World Y coordinate in tile "units".
+ * @param ground_vehicle Whether to get the Z coordinate of the ground vehicle, or the ground.
+ * @return World Z coordinate at tile ground (vehicle) level, including slopes and foundations.
+ * @see GetSlopePixelZ
+ */
+typedef int GetSlopeZProc(TileIndex tile, uint x, uint y, bool ground_vehicle);
 typedef CommandCost ClearTileProc(TileIndex tile, DoCommandFlag flags);
 
 /**