Center an entity on a given point.
@private
@param {object} game Required for game.entities.get().
@param {integer} entity The id of entity to center.
@param {object} point A point object <code>{"x": 50, "y": 50}</code> on which to center the entity.
function centerEntityOnPoint(game, entity, point) {
  var size = game.entities.getComponent(entity, "size");
  var position = game.entities.addComponent(entity, "position");
  position.x = point.x - (size.width / 2);
  position.y = point.y - (size.height / 2);
}