Reported directly to dev-team: vapor cloud dissipation didn't always
update vision properly.
Region removal affecting visibility needs to make two passes, the
first unblocking all no longer blocked spots, then the second deciding
whether spots are visible.
Attempting to do that in one pass was doing
| unblock <x1,y1>
| if cansee <x1,y1> whatever
| unblock <x2,y2>
| if cansee <x2,y2> whatever
and the cansee <x1,y1> test wasn't accurate if <x2,y2> blocked it and
hadn't been unblocked yet.
Testing with steam didn't seem to trigger the problem but with poison
vapor trail from green dragon breath did. The order of evaporation
mattered too; sometimes the single pass unblocking plus vision-testing
worked ok by coincidence.
9993fa97ec