Fix SectionPlane failure to clip certain objects #481
Description
Describe the bug
On xeokit 1.5.13
To Reproduce
Open our demo model,
create a section on Y axis:
sectionPlanesPlugin.createSectionPlane({
dir: camera.worldForward.map(val => -val),
pos: math.getAABB3Center(scene.getAABB())
});