[FEATURE REQUEST] Fallback indirect light cache for static lighting.

When working on low-end devices, like older mobiles, it is sometimes preferable to have a skylight and directional light which both have the mobility set to static. In the current version of the engine this means any dynamically movable object will become black outside of the Lightmass importance volume, which is because there are no indirect lighting volume samples.

Although it makes sense, this is not ideal for most games, because projectiles or physical objects fly out of the level. If they become black it is just too noticeable.

My suggestion is to have a fallback color, which the user can set. By default this can be black, so it won’t have an impact on current users. A secondary option could be to leave the last point sample state when it is outside of the Lightmass volume. This could even save performance since it can instantly stop blending when it is not inside the volume anymore.