Render Sky Plane

[ataulien]

In Gothic 1, the sky is rendered as a plane. This has been implemented in REGoth before, so the old implementation can serve as a reference:

https://github.com/REGoth-project/REGoth/blob/master/src/render/SkyRendering.cpp

In the new implementation, I suggest creating a component which takes care of rendering the sky. A plane-mesh is available inside bsf and can be retrieved like this:

  bs::HMesh planeMesh = bs::gBuiltinResources().getMesh(bs::BuiltinMesh::Quad);