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?? :/
newline
removing unused function to remove warning
commenting out unneeded things
moved SaveAIMap() to F9 (while traceAI is on)

getCtrlKeyDown() doesn't seem to be tracking Ctrl properly on Linux. Must
investigate later.
re-hiding Pathfinding's internal methods again
cleanup
Revert "removing unused function to remove warning"

This reverts commit a2e6d01eb325bd14e1f5420a5b9b21f88eb0cd3c.
Merge pull request #391 from hmaon/ai_fixups

survey function now uses Vol's new voxel exposure function