PlaneSeparator: don't overwrite plane offsets

When computing lastPlaneOffsets, the previous value was just being
overwritten. This is the incorrect behavior when given an actual desired
offset. Instead, the strip offset should grow from the given offset.
Fix cached plane logic

ChannelFiller and PlaneSeparator had erroneous logic in determining if
they had a cached plane or not (if the requested region contained the
cached region, the cached plane was returned - instead of the intended
reverse).

However, as there was no logic to properly extract desired sub-regions
from cached planes, cached planes will only be returned for now when
they match exactly.

This functionality will be reviewed in the future
(see https://github.com/scifio/scifio/issues/155)