Bumps forge2d to 0.12.2
Migration instructions:
The gravity and bullet are now field setters and getters, so if you
before had `setGravity(Vector2(0, -10))` then you now do `gravity =
Vector2(0, -10);` and if you previously had `body.setBullet(true);` you
now do `body.isBullet = true;`.