q3map2: do not use the new area light backsplash algorithm by Jelvan by default,...

q3map2: do not use the new area light backsplash algorithm by Jelvan by default, make it optional

NRC commit ed4c8c204443726e852a4c8927b3f8d2571cc522 replaced
the historic Quake 3 backsplash algorithm with an alternate
one by Jelvan:

> new area lights backsplash algorithm ( by Jelvan ),
> hijacking temp area lights ( to simulate volumetric
> behavior of source ones )

It suffers from many bugs and then only a few maps may benefit from it,
see: https://gitlab.com/xonotic/netradiant/-/merge_requests/210