Implement Sky

[ataulien]

Gothic supports two modes for it's sky:

• Huge flat plane with the sky-texture assigned (used by G1, selectable in G2)
  

• Dome-mesh with the sky-texture and some color-blending to simulate the horizon (only in G2)
  

Both of these have been implemented in old REGoth. The implementation decouples drawing from sky-logic and can be used as a reference.

• Logic: https://github.com/REGoth-project/REGoth/blob/master/src/content/Sky.cpp
• Rendering: https://github.com/REGoth-project/REGoth/blob/master/src/render/SkyRendering.cpp

We should probably use scene objects instead of rendering the meshes on our own though.

[ataulien]

Sky coloring and state interpolation has been implemented in PR #36. Rendering the skydome is still open.

[ataulien]

Closing this, as it has been split into #73 and #74.