Material Override in Model Node Settings is not un-doable

[kgschoueri]

The Material Override feature in the model node settings are non-reversible within the scene. Once override is 'true' the material changes to the model become permanent even when false is re-selected.

[gjcope]

Original model material properties are now cached and stored in v0.38.0 so override is un-doable by just turning the setting off.