(svn r2288) - CodeChange: protected the next batch of commands (41 so far, out of 115).

- CodeChange: changed the airport gui airport-type checking. Added function GetValidAirports() that returns bitmasked availibility, is also used for checking.
- CodeChange: to check tree-planting, 2 const arrays have been moved to table/tree_land.h (type and count)
- CodeChange: added IsTownIndex() in following of IsStationIndex(), etc.
- Fix (regression): road tunnels did not work anymore, forgot that their type was 0x200 (documented now)

table/tree_land.h

@@ -1,3 +1,6 @@
+#ifndef TREE_LAND_H
+#define TREE_LAND_H
+
 static const SpriteID _tree_sprites_1[4] = {
 	0x118D,
 	0x11A0,
@@ -5,6 +8,9 @@ static const SpriteID _tree_sprites_1[4] = {
 	0x11C6,
 };
 
+static const byte _tree_base_by_landscape[4] = {0, 12, 20, 32};
+static const byte _tree_count_by_landscape[4] = {12, 8, 12, 9};
+
 static const byte _tree_layout_xy[4][8] = {
 {9, 3, 1, 8, 0, 0, 8, 9},
 {4, 4, 9, 1, 6, 9, 0, 9},
@@ -211,3 +217,6 @@ static const PalSpriteID _tree_layout_sprite[164+(79-48+1)][4] = {
 	{     0x716,      0x716,      0x6f3,      0x6ec}, /* 30 */
 	{     0x716,      0x701,      0x6fa,      0x716}, /* 31 */
 };
+
+#endif /* TREE_LAND_H */
+