Blender Git Loki

Fixes for context updates of the node editor:
* If the node tree can be updated from context (tree has get_from_context callback defined), reset the pointers first to clear the editor path if no tree can be found.
* Stupid mistake: snode->from != snode->from is always false.
* Shader nodes context update: set the 'from' pointer to the active object, even if it doesn't have a material or node tree.