Decided to refactor a bit. Was using LLSD as an internal data

representation transferring ownership, doing data aggregation
in a very pedantic way.  That's just adding unneeded cost and
complication.  Used the same objects to transport data as are
collecting it and everything got simpler, faster, easier to
read with fewer gotchas.  Bit myself *again* doing the min/max/mean
merges but the unittests where there to pick me up again.  Added
a per-region FPS metric while I was at it.  This is much asked
for and there was a convenient place to sample the value.