Properly search for downstream mesh nodes in the material nodes

[jonathan-meier]

Both the PRTMaterialNode and the ArnoldMaterialNode need to find mesh nodes downstream the dependency graph. Currently, this is manually implemented, only following the plug connection chain via groupParts nodes (as these auxiliary nodes are inserted, when materials are assigned on the face level).

Task:

• Reimplement this search logic properly and with support for intermediate nodes other than groupParts nodes. MItDependencyGraph is likely to help.