insertBefore() is able to work on nodes from different documents, so on 
adding child or sibling it's possible that libxml2 switches doc pointer 
for a node, we need to update refcounts to cover that case.

The memory may not be freed until surface destruction otherwise. The PBO
creation code in surface_prepare_system_memory() also depends on the SYSMEM
location being recreated regularly, although arguably that's just a symptom of
the resource location management being somewhat broken.

This fixes a regression introduced by
1de6adfb.