various fixes for craft handling

- no "UFO lost" popup messages for UFOs attacking a base
- no "select new destination" message for crafts that are following said UFO
- properly delay alien missions in the event of a craft's destruction (not
just on disabling the ufo)
- don't allow "UFO BreakingOff" to override "UFO destroyed"
- end the retaliation if the base is destroyed (one way or another)
- delay spawning of the next UFO for this mission when carrying out an attack
against XCOM bases (no more attacks every 30 minutes)
- move "craft destroyed" checks out of dogfight and into geoscape to avoid cases
where "my interceptor has -105 health"
- add some safety in the UFO destructor to make it send any followers home
right sound for hovertank
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