EXT_swap_control redefines glXSwapIntervalSGI() as glXSwapIntervalEXT() on the
current drawable (if any), and glXSwapIntervalEXT() supports disabling vsync.

The swapchain was the last place using IParent, so this allows us to kill
IParent completely.

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.