Core/Entities: installing vehicle accessories

hash 747ea7c
3.3.5 branch
TDB 335.59

Add accessories with vehicle_template_accessory doesn't work for some vehicles, such as:

VehicleId: 341 - Flame Leviathan Seat (http://www.wowhead.com/npc=33114),

How to reproduce:
1 - Run:

DELETE FROM `vehicle_template_accessory` WHERE `entry` IN (33114);
INSERT INTO `vehicle_template_accessory` (`entry`, `accessory_entry`, `seat_id`, `minion`, `description`, `summontype`, `summontimer`) VALUES
(33114, 33142, 1, 1, 'Flame Leviathan Seat', 6, 30000),
(33114, 33143, 2, 1, 'Flame Leviathan Seat', 6, 30000);

Note that 33114 is an accessory of 33113.

2 - Spawn the seat with .npc add 33114 (note that it will install the accessories correctly)
3 - Spawn the boss .npc add 33113 (note that the seat will be empty-his accessories will despawn)

It seems to happen when a vehicle with its own accessories, is an accessory of another vehicle.

Start by applying the supplied vehicle_template_accessory SQL and reproducing the nested vehicle spawn sequence with .npc add 33114 and .npc add 33113. Trace the vehicle accessory handling for a vehicle that is itself an accessory, and consider the issue done when the seat's accessories remain installed after the boss is spawned.