MaterialNodeData.Initialize(INode) is now virtual, so that e.g. ColorNodeData can override and do initialization of it's own first. Implemented item reference checking for lists in NodeDrawer and MaterialNodeDrawer
Move time-dependent preview tracking into PreviewSystem
# Conflicts: # com.unity.shadergraph/Editor/Data/MasterNodes/PBRMasterNode.cs # com.unity.shadergraph/Editor/Util/UIUtilities.cs