(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/clear_map.h

@@ -126,7 +126,7 @@ static inline void MakeClear(TileIndex t, ClearGround g, uint density)
 {
 	/* If this is a non-bridgeable tile, clear the bridge bits while the rest
 	 * of the tile information is still here. */
-	if (!MayHaveBridgeAbove(t)) SB(_m[t].extra, 6, 2, 0);
+	if (!MayHaveBridgeAbove(t)) SB(_m[t].m6, 6, 2, 0);
 
 	SetTileType(t, MP_CLEAR);
 	SetTileOwner(t, OWNER_NONE);
@@ -134,7 +134,7 @@ static inline void MakeClear(TileIndex t, ClearGround g, uint density)
 	_m[t].m3 = 0;
 	_m[t].m4 = 0 << 5 | 0 << 2;
 	SetClearGroundDensity(t, g, density);
-	SB(_m[t].extra, 2, 4, 0);
+	SB(_m[t].m6, 2, 4, 0); ///< Clear the rest of m6, bits 2 to 5
 }