(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/landscape.cpp

@@ -415,7 +415,7 @@ void InitializeLandscape(void)
 		for (x = 0; x < maxx; x++) {
 			MakeClear(sizex * y + x, CLEAR_GRASS, 3);
 			SetTileHeight(sizex * y + x, 0);
-			_m[sizex * y + x].extra = 0;
+			SetTropicZone(sizex * y + x, TROPICZONE_INVALID);
 			ClearBridgeMiddle(sizex * y + x);
 		}
 		MakeVoid(sizex * y + x);