Shaders for mapname.rpak materials

[snake-biscuits]

To get lighting in Titanfall 2 & Apex, you need to use wld matls from .rpak
Currently, making your own is way too much work
So, it'd be nice to let mappers target the .rpak of an existing map

We could reverse the flags for each material from .bsps
IIRC there's something like that in MRVN-VMT already

Tho we might need something in Radiant to help mappers avoid mixing .rpaks

I would also like to make up "theme" reference sheets for picking a .rpak

Obviously a full map release should ship with it's own .rpak, but sharing is ok for alpha