Camera follow and/or screen transitions

[rustrix]

Depends on how MetSys handles screens: #30

• [ ] At least have the camera follow the character similar to EnviroMetal.

Optional features:

• [ ] Camera becomes offset when the character is moving, to show more of the area in front of the character than behind.
• [ ] Camera is locked into the level boundaries, to avoid showing out-of-bounds areas. (implemented in EnviroMetal)
• [ ] If there will be hard barriers between screens (like Celeste), add screen transition animation to keep it from being jarring (fade-to-black)

[DaltonSW]

MetSys touts a 'screen transition' system, so I think that kind of thing will be reasonably handled for us.

Camera following can maybe be yoinked directly from EM

[DaltonSW]

Both are handled by MetSys and PR #91