Fixes #336

This fix accomplishes two things:

1. This fixes a layout bug where some graphs will have an incorrect node ordering. This happens because the original ordering is already optimal (zero crosses) but the cross minimization in order/index.js will permute the nodes several times anyways even though it can't decrease the cross count.

2. This is a perf optimization. If cross count gets to 0 then we should stop the loop. I noticed that running make bench shows an increase in the ops/sec for the order phase of around 700 to 840.

Note that this breaks atleast one case of compound graphs (the failing tests) and I'm not entirely sure why compound graphs are required to go through the sweepLayer pass in order to get a proper layout. It seems that the cross minimization should be separated from the compound graph layout. I did notice that moving the cross minimization check to within the for-loop (after sweepLayer) fixes the test. That may be one possible fix and I'll put up a separate PR. However, it does not fix the original bug that I was trying to fix.

Before:

After:

Perf before:

          longest-path ranker:    469,894.67 ops/sec ± 0.12% ( 94 run(s) sampled)
            tight-tree ranker:    125,875.60 ops/sec ± 0.08% (101 run(s) sampled)
       network-simplex ranker:     26,225.72 ops/sec ± 0.13% ( 95 run(s) sampled)
                       layout:        707.90 ops/sec ± 1.39% ( 86 run(s) sampled)

Perf after:

          longest-path ranker:    462,488.92 ops/sec ± 0.14% ( 95 run(s) sampled)
            tight-tree ranker:    125,875.39 ops/sec ± 0.09% ( 98 run(s) sampled)
       network-simplex ranker:     26,180.47 ops/sec ± 0.18% ( 97 run(s) sampled)
                       layout:        847.17 ops/sec ± 1.26% ( 87 run(s) sampled)

The main issue with this fix is that it breaks compound graph layout. It breaks a unit test, and I confirmed by rendering the graph from that unit test using test/console.html.

The test is layout-test.js: can layout subgraphs with different rankdirs. And the graph looks like:

    g.setNode("a", { width: 50, height: 50 });
    g.setNode("sg", {});
    g.setParent("a", "sg");

I'm not familiar enough with the compound graph layout additions to order.js to know if its actually essential that we run the cross minimization iterations in order to layout compound graphs, or if there's a way to do what we need for compound graphs but still return early if cross count is 0 before doing all those iterations. I'm going to look at putting up another PR that tries to meet in the middle by returning early after 1 iteration, which I think fixes compound graph layout but also gives perf benefits of not running all the iterations.