Fix target FX on retreating units (#2166)

## Description:

Retreating naval invasions spams the target FX each frames.
Changes: filter out retreating invasions.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

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

IngloriousTom

src/client/graphics/layers/FxLayer.ts

@@ -122,7 +122,7 @@ export class FxLayer implements Layer {
         const my = this.game.myPlayer();
         if (!my) return;
         if (unit.owner() !== my) return;
-        if (!unit.isActive()) return;
+        if (!unit.isActive() || unit.retreating()) return;
         if (this.boatTargetFxByUnitId.has(unit.id())) return;
         const t = unit.targetTile();
         if (t !== undefined) {