bit of work cleaning up default target and clearin blocks.  working on a
default list of skippable world-gen blocks for when checking depth/block
heights
more minor updates trying to track down infinite loop or just out of
memory...and then other times it works fine.

need to force under-fill on structures.  need to check 'skippable
blocks' list (just use clearable blocks list?) when checking
block-target height for intial placement.

need to verify block under outside edge of border is a valid target
block and not air