Change "Force default shader" on proxies

[Twist]

Hello. While I like the "Force default shader" option on proxies I wish you would have implemented a slot for shaders instead. That way it's possible to have different shader overrides on different proxies. The current way is too rigid.

Thank you

[hermit.crab]

Currently, the default shader refers to the one assigned to the VRayProxy itself. Otherwise, I've mentioned the idea to the product owner.