Implement vertical fog gradiant for skyboxes.
World fog alone doesn't suffice since it isn't applied to the skybox, as a result scenes with fog and bright skyboxes look odd.
Skybox fog compensates for this by using a vertical gradiant which is applied over the skybox. There are three constants which
affect the skyfog gradient. `skyFogColor', used to affect the color of the fog, `skyFogCloudColor', used to affect the color of
fog near the clouds and `skyFogDensity', used to indicate how dense the fog should be.