- Fixed a bug on World.Clear().
- Implemented World.Update().
- Basic circle physics added.
- Added physics grouping, which decouples events
from PhysicsComponent.
- Some tweakes to game play (like bullet spawning position, for instance).
- Re-implemented update method on World.cs.
- Changed Transform.Position from a Vector2 to a Vector3 (enables Z
  sorting on draw).
- Added a Speed component to Physics Component.
- Renamed Viewport class to Renderer.
- Hopefully fixed 2D Hashing on World class.