to three conversion: avoid RangeErrors when dealing with large arrays

[uqee]

Current implementation: destinationArray.push.apply(destinationArray, sourceArray); fails to process large arrays (at least it fails at ~130k elements for me) with the RangeError: Maximum call stack size exceeded. Since neither threePts nor threeFaces is actually used later, they just duplicate memory consumption and can be removed in favor of direct geometry.vertices and geometry.faces manipulation.

[pboyer]

Thanks @uqee! Looks good.