better transport ship spawn (#587)

## Description:

Taken from PR #506

Improve transport source tile by considering border extremums

Only calculate better spawn tile for humans, and have the sender
calculate it and send the src tile in the intent for better performance.

## 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>

tests/Attack.test.ts

@@ -19,9 +19,9 @@ let defender: Player;
 let defenderSpawn: TileRef;
 let attackerSpawn: TileRef;
 
-function sendBoat(target: TileRef, troops: number) {
+function sendBoat(target: TileRef, source: TileRef, troops: number) {
   game.addExecution(
-    new TransportShipExecution(defender.id(), null, target, troops),
+    new TransportShipExecution(defender.id(), null, target, troops, source),
   );
 }
 
@@ -97,7 +97,7 @@ describe("Attack", () => {
     constructionExecution(game, defender.id(), 1, 1, UnitType.MissileSilo);
     expect(defender.units(UnitType.MissileSilo)).toHaveLength(1);
 
-    sendBoat(game.ref(15, 8), 100);
+    sendBoat(game.ref(15, 8), game.ref(10, 5), 100);
 
     constructionExecution(game, defender.id(), 0, 15, UnitType.AtomBomb, 3);
     const nuke = defender.units(UnitType.AtomBomb)[0];