Fog visibility freezing

This hides a few effects that shouldn't be visible behind the fog:
* Explosions / gunfire
* Infantry death animations
* Smoke

The state of resource fields, smudges, and bibs (if enabled) are frozen as they were last seen.

The repair and powerdown icons are hidden for cloaked units (or any other IVisibilityModifier, including HiddenUnderFog if enabled).

I have dropped the FrozenUnderFog improvements for now, as this will need some low level changes for faking the order targeters against frozen buildings where the backing actor was destroyed.