Feature: allow setting a custom terrain type to define highest peak

At least, TGP will try to reach it. It heavily depends on the map
if it is reachable at all. But for sure it will do its atmost to
get there!

src/table/settings.ini

@@ -266,7 +266,7 @@ from     = SLV_97
 guiflags = SGF_MULTISTRING | SGF_NEWGAME_ONLY
 def      = 1
 min      = 0
-max      = 4
+max      = 5
 interval = 1
 str      = STR_CONFIG_SETTING_TERRAIN_TYPE
 strhelp  = STR_CONFIG_SETTING_TERRAIN_TYPE_HELPTEXT
@@ -2463,6 +2463,17 @@ strhelp  = STR_CONFIG_SETTING_EXTRA_TREE_PLACEMENT_HELPTEXT
 strval   = STR_CONFIG_SETTING_EXTRA_TREE_PLACEMENT_NO_SPREAD
 cat      = SC_BASIC
 
+[SDT_VAR]
+base     = GameSettings
+var      = game_creation.custom_terrain_type
+type     = SLE_UINT8
+flags    = SLF_NOT_IN_SAVE | SLF_NO_NETWORK_SYNC
+guiflags = SGF_NEWGAME_ONLY
+def      = MAP_HEIGHT_LIMIT_AUTO_MINIMUM
+min      = MIN_CUSTOM_TERRAIN_TYPE
+max      = MAX_MAP_HEIGHT_LIMIT
+interval = 1
+
 [SDT_VAR]
 base     = GameSettings
 var      = game_creation.custom_sea_level