Add different skies to scenic

[pmiddend]

Possible skies are:

• Skydome
• Skybox (maybe using cube maps)
• Gradient?
• Constant color clearing

Skies are a global scene property and should be initialized in the prototype ctor. Skydomes/boxes/gradients can be drawn after all other objects, clearing has to occur before everything else.

Make a variant containing all sky types.