Fix clipping of nodes when CLIP_OUTSIDE is set

[rhuitl]

I was trying to use the CLIP_OUTSIDE mode but only got very few points to show up. I tracked it down to the shouldClip method which seems to return the wrong value. From its call

if (
  node.level > maxLevel ||
  !frustums[pointCloudIndex].intersectsBox(node.boundingBox) ||
  this.shouldClip(pointCloud, node.boundingBox)
) {
  continue;
}

and the early check in the method

if (material.numClipBoxes === 0 || material.clipMode !== ClipMode.CLIP_OUTSIDE) {
  return false;
}

I see that it should return true only when a node should be invisible (clipped). However in the checks further down it would return false when the node does not intersect with any of the clip boxes.

[hccampos]

Thanks @rhuitl ! Merged and released to npm, along with a few other fixes.