Legends of Aethereus: Fix lighting flicker + better full screen quad test

- Flickering was due to using an uninitialised variable when drawing a
  full screen quad. I'm an idiot for missing that one.
- Testing output w component seems more reliable than testing input
  positions to determine if drawing a fullscreen quad, and requires less
  branches.
Legends of Aethereus: Add skybox shader
Legends of Aethereus: Push skybox depth to infinity