Arnold: avoid MEL script variable shadowing

[jonathan-meier]

When generating Arnold material for multiple models, we always reuse the same variables in the emitted MEL script, which leads to a variable shadowing warning.

Task:

• Find a way to avoid these warnings.

Note: One possibility we might try is to encapsulate the MEL script logic in a command using C++, which as a bonus might even execute faster than the MEL script.