Weapon#createBullets()

This method performs two actions: First it will check to see if the Weapon.bullets Group exists or not,
and if not it creates it, adding it the group given as the 4th argument.

Then it will seed the bullet pool with the quantity number of Bullets, using the texture key and frame
provided (if any).

If for example you set the quantity to be 10, then this Weapon will only ever be able to have 10 bullets
in-flight simultaneously. If you try to fire an 11th bullet then nothing will happen until one, or more, of
the in-flight bullets have been killed, freeing them up for use by the Weapon again.

If you do not wish to have a limit set, then pass in -1 as the quantity. In this instance the Weapon will
keep increasing the size of the bullet pool as needed. It will never reduce the size of the pool however,
so be careful it doesn't grow too large.

You can either set the texture key and frame here, or via the Weapon.bulletKey and Weapon.bulletFrame
properties. You can also animate bullets, or set them to use random frames. All Bullets belonging to a
single Weapon instance must share the same texture key however.