We’re looking for cases where place(a, b, c) will place c so it
intersects with a or b.

The old place(a, b, c) function would sometimes place c so it overlapped
with a and/or b, if c had a much larger radius than a and b.

This version doesn’t seem to.

As per discussion on #111.

This commit also updates the tests in find-place-bugs.js to
use the new function, and updates the intersection test in
siblings-test.js to match

Note that flare-test.js fails. I haven’t investigated why: I hope it is simply
a result of the changes in numerical precision causing slightly different
circle placements.

Since the values that had been divided by scale = sqrt(dc) are
only being used in pairs, we can avoid the square root and just
divide by dc instead.

This should help with the precision, marginally, I hope.