Renamed original Fog to DeferredFog (because it is only deferred effect)
Added Fog effect which modifies RenderSettings before rendering and restores after rendering

Related to #512

Implementation consists of three parts

1. Loading base fog values from RenderSettings in PostProcessEffectSettings.Reset.
2. Applying fog to RenderSettings in OnPreCull
3. Restoring original RenderSettings fog values in OnPostRender

Loading base values is necessary for overrides to work properly. We want overrides to work 'on top' of settings which are in RenderSettings.

What works

• Fog settings
• Overrides
• Consistent fog on transparent and opaque materials
• Different fog on multiple cameras
• Fog blending between volumes
• Preview in scene view (image effects/fog)

Possible issues

• Baking lights?
• GI (Enlighten)?

Not sure how light baking and Enlighten GI works, it might use fog from global RenderSettings for some calculations.