Fixes #198 - only dirty nodes were being used as starting point for PreviewMode propagation

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/PreviewManager.cs

             PropagateNodeSet(m_DirtyPreviews);
 
             m_NodesWith3DPreview.Clear();
-            foreach (var index in m_DirtyPreviews)
+            foreach (var node in m_Graph.GetNodes<AbstractMaterialNode>())
-                var node = (AbstractMaterialNode)m_Graph.GetNodeFromTempId(m_Identifiers[index]);
                 if (node.previewMode == PreviewMode.Preview3D)
                     m_NodesWith3DPreview.Add(node.tempId.index);
             }
         void DestroyRenderData(PreviewRenderData renderData)
         {
             if (renderData.shaderData != null
-                && renderData.shaderData.shader != null 
+                && renderData.shaderData.shader != null
-            
+
             if (renderData.shaderData != null && renderData.shaderData.node != null)
                 renderData.shaderData.node.onModified -= OnNodeModified;
         }