(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/map.h

@@ -23,7 +23,7 @@ typedef struct Tile {
 	byte m3;
 	byte m4;
 	byte m5;
-	byte extra;
+	byte m6;
 } Tile;
 
 extern Tile* _m;