Change sea_route to map overlay without face

Change sea routes from ordinary ground tiles to an invisible overlay.
Because they have no face, the new sea routes are never sent to the
client and are only visible in the map editor.

Add several varieties of the sea_route tile, in case multiple sea routes
overlap on the same map square.

ground/sea.arc

@@ -31,21 +31,28 @@ anim_speed 5
 randomitems sea_fish
 end
 Object sea_route
-name sea
-face sea.111
-animation sea
-smoothface sea.111 sea_S.111
-smoothface sea.112 sea_S.112
-smoothface sea.113 sea_S.113
-smoothface sea.114 sea_S.114
-move_block all -boat
+name sea route 1
+face blocked_cyan.111
+invisible 1
+no_pick 1
+end
+Object sea_route_2
+name sea route 2
+face blocked_lime.111
+invisible 1
+no_pick 1
+end
+Object sea_route_3
+name sea route 3
+face blocked_magenta.111
+invisible 1
+no_pick 1
+end
+Object sea_route_4
+name sea route 4
+face blocked_red.111
+invisible 1
 no_pick 1
-is_water 1
-is_floor 1
-smoothlevel 20
-client_anim_sync 1
-anim_speed 5
-randomitems sea_fish
 end
 Object sea_nw
 name sea