Add support for bridge construction

Each movement class is given a crude transientDomain graph.  Whenever a bridge state
changes, the domain index rebuilds the relevant cells, setting its domain and creating
a transient domain connection.

This graph is searched as a fallback if the straightforward domain comparison is false.
Behavior needs to be double-checked, and ideally destructing connections would be supported,
but so far performance and behavior on allies-02 seems good.

OpenRA.Mods.RA/Bridge.cs

@@ -218,6 +218,12 @@ namespace OpenRA.Mods.RA
 			foreach (var c in TileSprites[currentTemplate].Keys)
 				self.World.Map.CustomTerrain[c.X, c.Y] = GetTerrainType(c);
 
+			// If this bridge repair operation connects two pathfinding domains,
+			// update the domain index.
+			var domainIndex = self.World.WorldActor.TraitOrDefault<DomainIndex>();
+			if (domainIndex != null)
+				domainIndex.UpdateCells(self.World, TileSprites[currentTemplate].Keys);
+
 			if (LongBridgeSegmentIsDead() && !killedUnits)
 			{
 				killedUnits = true;