Review/unify dithering and fading systems

High priority item:

"Hi guys, I was looking to a GPU capture of our foliage and all draws were set with the KILL_ENABLE register enabled.
This register force off Early Z rejection since the GPU has no guarantee the Z will be actually written (so it postpone comparison and write to Late Z).
The shader compiler turn on this register since it found the pixel discard instruction injected by the USE_DITHERED_LOD_TRANSITION_FOR_INSTANCED compile directive.
On Mobile platforms this directive is enabled only if the material has been set with the DitheredLODTransition, however PC and consoles it's always set to true."

Lower prio:

There are multiple competing systems that control fading and dithering between LODs, over distance and special cases. This currently leads to various graphical and performance issues. Review the existing cases and see if we can unify any of the systems.