In complex materials graph and shader graph edge iteration can be
extremely slow, because some edges may be visited many times
unnecessarily. This is especially noticable in two functions:
ShaderGraph::addUpstreamDependencies and ShaderGraph::optimize() .

GraphIterator and ShaderGraphEdgeIterator classes iterate over DAGs
without marking nodes as visited, which may lead to exponential
traversal time for some DAGs:
https://stackoverflow.com/a/69326676

This patch adds an option to skip visited edges in GraphIterator and
modifies ShaderGraphEdgeIterator so that each edge is visited only once.