Closes#75.
The outer row of a fill sat 1.5 spacings from the bed edge where the rule says
half a spacing, staggered rows started a full pitch in, and the far edge had
clumps hanging 13cm outside a bed nothing clips them to. Centre the lattice and
inset each edge by radius - spacing/2.
Also fixed here: a region that misses the object entirely planted plops metres
off the bed (clampTo inverts rather than empties, which the old loop handled
implicitly and the counted lattice did not), non-finite region bounds now give
ErrInvalidInput instead of a raw store error or a silent no-op, and the lattice
size is derived before it is built so an oversized fill is refused without
allocating what it is rejecting.
Five Gadfly rounds; the substantive findings were the finiteness guard and the
allocate-before-cap ordering.
steve