Enable garrisoning on gates / fix movement of units with visible garrison points (2nd commit)
This addresses two related issues:
- Units visibly garrisoned on gates keep the gate open.
- Units visibly garrisoned on entities keep their pathfinding blocker flags.
De-activate the obstruction of visibly-garrisoned entities, fixing the 2nd issue.
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
Differential Revision: https://code.wildfiregames.com/D2775