Instead of shuffling all the shells once for each tick used,
shuffle only those shells actually used once per tick. This
should save us more time in common cases where we need
to only check a few shells to find a place to settle.
- Always prefer closest place first, then lowest height.
- Simplify search order; use a pre-built order instead of scan.
- Randomize search order only once per tick, if used.
The old method was designed for 0.4 compatibility. Since 5.0
there appears to be a flag we can just check to see if the item
is still moving according to physics or settled.
