PlaneSeparator: fix lastPlane population
PlaneSeparator: fix lastPlane population

The plane offsets for strips were being used incorrectly - storing the
last read strip offset and then adding (i * striplength) for the next
strip's offset. Only one or the other should be done - i * strip length
+ the base offset, or last offset + strip length.

Updated to use base offset + i * strip length, which should improve
performance in low memory situations.

scifio/scifio