Allow clip surface convex hulls to overlap

[magenta404]

This was a known issue since inception of the feature and is documented. Adding it here too so people see it.

The second pass removes the clip surface data which is causing the overlap issue. The only solution I can think of is rendering to a temporary texture and combining it with the main data texture.

Will see how many people request it before looking at it further.

[magenta404]

#879 provides an alternative to this problem which will be out by 4.13.