FadeMod corner cases.

[Hoikas]

The fade mod has the following corner cases and design issues:

• plFadeOpacityMod back fade mods need to have a coordinate interface, otherwise, the fade is calculated from the LOS to the origin. Currently, this is not enforced.
• The simple distance tip is wrong. plDistOpacityMod fades objects out as you move closer.
• Given the previous, we need a way to do fade in using icicles.
• kTrackCamera is not exposed for plDistOpacityMod based faders - meaning the avatar transform is used for fades.
• The warning about distances is erroneously displayed in LOS mode.

[dpogue]

Fading things in probably requires using LOD (setting the fMinDist and fMaxDist in the plIcicles for each object)

[dpogue]

Took a bit closer look at how this is handled in PlasmaMax.

• plFadeOpacityMod is working fine (probably with the caveat you mentioned about needing a CI)
• plDistOpacityMod seems to only ever be used by GZ markers, and in PlasmaMax is actually labelled as "GZ Fade"
• Fading objects in over distance is done by setting in MinDist and MaxDist on the geoSpan for the object

Maybe we just want to change the "Simple Distance Fade" mode to set the icicle stuff, and leave the complicated distance stuff to plDistOpacityMod?