fix min/max header not being correct due to improper multithreading
fix min/max header not being correct due to improper multithreading

# Conflicts:
#	Converter/src/chunker_countsort_laszip.cpp

potree/PotreeConverter,potree/PotreeConverter,potree/PotreeConverter,potree/PotreeConverter,potree/PotreeConverter,potree/PotreeConverter,potree/PotreeConverter