Make FOG_ENABLED a shader_def instead of material flag

[IceSentry]

Objective

• If the fog is disabled it still generates a useless branch which can hurt performance

Solution

• Make the flag a shader_def instead

Testing

• I tested enabling/disabling fog works as expected per-material in the fog example
• I also tested that scenes that don't add the FogSettings resource still work correctly

Review notes

I'm not sure how to handle the removed material flag. Right now I just commented it out and added a not to reuse it instead of creating a new one.

[robtfm]

makes sense, fog tends to be all or nothing so this is unlikely to make more shader variants in any real world scenarios, and saves some registers when not used.

i think keeping the other flags unchanged and commenting the old fog flag as unused is the right call as well.

[pcwalton]

This wasn't done the right way, I don't think. What I suggested was to make the FOG_ENABLED shader def contingent on whether the view has fog, not whether the material had a flag that disables it.