Rework MeshObject material manipulation about blenderproc

[cornerfarmer]

Hey @AndreyYashkin,

thanks a lot for the issue and the PR! I currently dont fully understand the use-case of the new functionality. However, at the same time, the new feature adds quite a bit of complexity to the material setter/getter and my fear is that this might lead to non-intuitive behavior.

I do understand that you would like to change the material of objects with linked data.
So what speaks against the following workflow:

-> Every time a user sets a material via obj.set_material() the respective material slot gets set to OBJECT

This makes sure that the material is always set only for the specific object instance.
If the user wants to set a material across multiple objects, the user needs to call obj.set_material() inside a for obj in objs loop. But I dont think that is much of a problem.

What do you think about this solution? In my eyes this would be beneficial as it reduces the complexity quite a bit.

[AndreyYashkin]

Sorry for the late response.

1. From my perspective warping Blender functionality does not lead to any non-intuitive behavior. In contrast, you may try the same operations inside Blender without coding and get the same result. In my PR I added only additional functionality to get actual visible material which is not obvious to get using code in one case, while inside Blender you can see it with your eyes.

2. Every time a user sets a material via obj.set_material() the respective material slot gets set to OBJECT

Well, I think that it can work too.