Generals: Multiple units can collect from a supply dock at once

[charliefoxtwo]

Only one unit should be able to collect from a given supply source at a given time, but in OpenSAGE multiple supply vehicles can collect simultaneously.

Guessing we'll need to add logic similar to what is in DockUpdate:

OpenSAGE/src/OpenSage.Game/Logic/Object/Update/DockUpdate.cs

Lines 101 to 117 in 48cad9a

	if (_unitsApproaching.Count > 0)
	{
	var aiUpdate = _unitsApproaching.Peek();
	switch (aiUpdate.SupplyGatherState)
	{
	case SupplyAIUpdate.SupplyGatherStates.EnqueuedAtSupplyTarget:
	aiUpdate.SupplyGatherState = SupplyAIUpdate.SupplyGatherStates.StartDumpingSupplies;
	break;
	case SupplyAIUpdate.SupplyGatherStates.FinishedDumpingSupplies:
	_gameObject.ModelConditionFlags.Set(ModelConditionFlag.DockingActive, true);
	aiUpdate.SupplyGatherState = SupplyAIUpdate.SupplyGatherStates.SearchingForSupplySource;
	_unitsApproaching.Dequeue();
	MoveObjectsForward();
	break;
	}
	}