Small optimizations in air and camera visibility init

 Optimizations in insignificant procs (less than 2% of total self time each (according to the june 7 profile), and the changes probably don't have a huge impact on them.

/turf/simulated/proc/update_air_properties()
 - Changed border lists to use |= rather than -= then +=.

/datum/camerachunk/New()
 - Uses block() rather than range() and a coordinate check

Comment (copied to two places) outlining a potential optimization. (potential as in "needs to be tested whether it actually makes things faster, or creates massive lag")