Balance, bug fixing, and new art

- Add a doughnut bullet for the helicopter boss (wave 5)
- Made most upgrades slightly more expensive

- Increased the amount of enemies that spawn per level (was 2x the level, now it is 3x)

- Nerfed attack speed upgrade from 5% to 3%, it was just way too over powered at a certain point.

-More attempts to 'Warm Up' particle effects before they are displayed to the player for the first time, the first time a particle loads it seems to cause small freezes/lag. The second playthrough it won't be present .. so hopefully this works to combat this.