Blender Git Loki

Code cleanup: remove WIP code came from the GSoC branch

DAG node tagging was rather an experiment to make derived render working.
However, it ended up in a whole can of worms and need to be re-considered.
It is likely that regular object update tagging and scene update routines
are to be used for this.

Meanwhile no need to keep extra field in dag node. Would save us the whole
byte of the struct which we can use for other purposes meanwhile.