survey function now uses Vol's new voxel exposure function

cover AI was completely broken because I reversed the signs between civilians and aliens... I really globbed up
different trace message
fixing reapers
another major AI fix + visualization (Ctrl-F10)
AI speedups (thanks, Vol);blazing fast screenshots

After figuring out the SDL_Surface format that lodepng can understand,
I was able to convert the screenshot function to use an SDL_BlitSurface()
call instead of a giant for() loop.
show units on levels below in AIMap visualization
small tweaks to visuals
distanceSq() was broken! also banished some sqrt()

seriously, distanceSq()
I can't even

also added MAX_VIEW_DISTANCE check to survey...() function for better AND
faster AI
oh yeah, free the surface
why abs() a value I'm about to square?? :/
Merge branch 'ai_fixups' of github.com:hmaon/OpenXcom into ai_fixups
no volume changes work with native midi

f@#$ it.