(#18): potential wall collision fix

frontend/core/src/com/game/tankwars/model/Tank.java

@@ -103,7 +103,7 @@ public class Tank {
             chassisSprite.flip(true, false);
             directionLeft = false;
         }
-        if (chassis.getPosition().x <= VIEWPORT_WIDTH - TANK_WIDTH){
+        if (chassis.getPosition().x <= TankWarsGame.GAMEPORT_WIDTH - TANK_WIDTH){
             setPosition(newPos);
             chassis.setTransform(newPos.x, newPos.y + 0.11f, angle);
             chassisSprite.setRotation(angle);