Consider zoom when culling billboards.

screenScale represents the current zoom factor, but may need some
revision because I'm not sure if what it's doing is technically correct.
It is used to multiply the culling distance for cells, as well as the
fading distance.
Added comment.
Fix engine API copy/paste errors (typo).
added missing initialization and set pointer to NULL after deletion
moved filesystem init further up, to be able to log errors earlier
Merge pull request #144 from eightyeight/ground-cover-lod

GroundCover fading when zoomed in
Merge pull request #241 from just-bank/engine-api-fixes

Engine API fixes
Merge pull request #332 from tdev/beamng

some small patches