Update doxygen config file template to doxygen 1.8.6

Patch contributed by Johannes Brunen, many thanks!
remove build option OSG_ENABLE_PAR_PARTITION_DRAWING

No code appears to be guarded by this any longer.
remove build option OSG_ENABLE_NEW_SHADER

There is only some code guarded by OSG_NEW_SHADER_DISABLE which is not
set anywhere AFAICS, remove it too.
fixed: propagate BackgroundLoader changes, so models loaded propagate in the cluster
added: make graph ops run by BackgroundLoader user changeable