Avoid call stack overflow on deeply nested splits

[EpicDima]

Fixes #215.

With this PR nested splits no longer recurse. The outermost split keeps the pending halves on an explicit stack and
triangulates them in a loop. Halves come off it in the same depth-first order as before, so the output is the same, in
the same order. earcut() and earcutLinked are unchanged, and the new code runs only after a split diagonal has
been found.

	main	this PR
example from the issue, 32,768 vertices	RangeError	same triangles as main with --stack-size=7000
bench/bench-tiles.js, median of 6 runs	363 ms	363.5 ms

The output is also identical to main's on the test fixtures, all polygons of bench/tiles-fixture.bin and about
228,000 random polygons. The new test runs in a worker with a 0.35 MB stack, so that it overflows on main quickly and
reliably.