Extract Sky drawing from volumetric clouds class.

[fede-vaccaro]

Currently, CloudsModel and VolumetricClouds classes together are responsible for drawing the sky (also, previously they were merged). There is a stub Skybox class: it's better to reduce the responsability of these classes and put it into Skybox. It will be better having sky and clouds textures separated, especially when we will implement the interaction between the terrain and the clouds.

[fede-vaccaro]

#6