worldToLocal(x, y, v) {
		// TODO memoization for one set of coords (multitouch)
		v = v || vector2dPool.get();
		v.set(x, y);
		if (!this.currentTransform.isIdentity()) {
			this.currentTransform.apply(v);
		}
		return v.sub(this.pos).add(game.world.pos);
	}
convert the given world coordinates into "local" (screen) coordinates
@param {number} x
@param {number} y
@param {number} [v] - an optional vector object where to set the converted value
@returns {Vector2d} a vector with the converted local coordinates