(svn r8050) -Codechange: Rename map member extra to m6, since its usage has been widden.

Only doc landscape_grid.html has been updated, landscape.html will be done latter.
-CodeChange: Replace a direct access to m6 with SetTropicZone in InitializeLandscape, as it is the only part of m6 still untouched without the brute-forced m6=0

src/tile.h

@@ -105,7 +105,7 @@ static inline bool IsTileOwner(TileIndex tile, Owner owner)
 static inline void SetTropicZone(TileIndex tile, TropicZone type)
 {
 	assert(tile < MapSize());
-	SB(_m[tile].extra, 0, 2, type);
+	SB(_m[tile].m6, 0, 2, type);
 }
 
 /**
@@ -117,6 +117,6 @@ static inline void SetTropicZone(TileIndex tile, TropicZone type)
 static inline TropicZone GetTropicZone(TileIndex tile)
 {
 	assert(tile < MapSize());
-	return (TropicZone)GB(_m[tile].extra, 0, 2);
+	return (TropicZone)GB(_m[tile].m6, 0, 2);
 }
 #endif /* TILE_H */