Multi src astar (#594)

## Description:
Samples border shore tiles and uses multi-a* for determining the
transport ship spawn cell.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

<DISCORD USERNAME>
evan

---------

Co-authored-by: evan <openfrontio@gmail.com>

src/client/graphics/layers/RadialMenu.ts

@@ -387,8 +387,9 @@ export class RadialMenu implements Layer {
         // BestTransportShipSpawn is an expensive operation, so
         // we calculate it here and send the spawn tile to other clients.
         myPlayer.bestTransportShipSpawn(tile).then((spawn) => {
-          if (spawn == false) {
-            return;
+          let spawnTile: Cell | null = null;
+          if (spawn !== false) {
+            spawnTile = new Cell(this.g.x(spawn), this.g.y(spawn));
           }
 
           this.eventBus.emit(
@@ -396,7 +397,7 @@ export class RadialMenu implements Layer {
               this.g.owner(tile).id(),
               this.clickedCell,
               this.uiState.attackRatio * myPlayer.troops(),
-              new Cell(this.g.x(spawn), this.g.y(spawn)),
+              spawnTile,
             ),
           );
         });