Enable garrisoning on gates / fix movement of units with visible garrison points
This addresses two related issues:
- Units visibly garrisoned on gates keep the gate open.
- Units visibly garrisoned on entities keep their pathfinding blocker flags.
Remove the block-movement flag from visibly garrisoned entities.
Keep a list of entities that cannot move and thus should not count towards gate-opening logic.
Packing logic is kept separate: it is more related to entities having 'alternate forms' with different capabilities than being currently incapable of moving.
Based on work by temple
Comments by: Freagarach
Tested by: Nescio
Differential Revision: https://code.wildfiregames.com/D1418