Allow proper collision on non-rectangular shaped elevators.

[p4plus2]

This change breaks the bounding box into strips rather than a single bounding box. This gives a more expected behaviour to non-rectangular platforms. Due to the "big margin" check sometimes it is difficult to fall through a one block gap, perhaps it is worth making the margin a config value (how was the value 150 chosen anyway?).

Two notes:

1. I am not sure how one would "upgrade" existing elevators which were moving on world load. One method could be a hack like:

if (tagCompound.hasKey("bminX")) {
    bounds.clear();
    int bminX = tagCompound.getInteger("bminX");
    int bminZ = tagCompound.getInteger("bminZ");
    int bmaxX = tagCompound.getInteger("bmaxX");
    int bmaxZ = tagCompound.getInteger("bmaxZ");
    bounds.add(new Bounds(bminX, bminZ, bmaxX, bmaxZ));
}

2. I have never really done any code in java let alone a minecraft mod, so this approach may not be implemented optimally, this approach was done more or less by vaguely emulating the style of surrounding code where obvious. I am open to changing or correcting the implementation if there is a better way to do it.

[p4plus2]

Noticed a small change that should also be made if (bounds == null) { should be if (bounds.isEmpty()) Strictly speaking it makes no difference and the check could probably be fully removed as far as I can tell, but proper error checking isn't a bad thing.