Performance optimizations

[easyaspi314]

Contributes to #141 on the lower level.

This greatly reduces memory and CPU usage without touching any algorithms.

• Change most Vector2 operations to be destructive ( += ) if possible, returning the this pointer instead of a new vector.
  • Non-destructive ones now use static methods
  • Some operations were shifted to allow this to work.
• Add more complex Vector2 functions that do not use temporaries (distance, angle, direction)
• Convert SpriteTransformer to static methods and directly pass the Canvas to the callback instead.
• Make Paint brushes static if possible
• Convert some ArrayLists to HashSets if applicable, allowing O(1) contains lookup
• Avoid creating UnmodifiableList/UnmodifiableCollection instances, instead return the list directly.

[easyaspi314]

I'm going to add a few more optimizations since I have these changes already.

[mjaun]

Looks very good! Thanks a lot for your improvements!