[Bf-viewport] Unified Principled BSDF and Eevee PBR

[Dalai Felinto]

I see two different points here.

1) Use the same material output node for Cycles and Eevee
2) Use the same shader node for Cycles Principled and Eevee Metallic

1) Use the same material output node for Cycles and Eevee
==============================================
The current design was to allow for material tweaks for different engines.

For now you would do it by having an Eevee and a Cycles output node
side by side. An alternative is to have a switch node in the nodetree
that runs different options depending on the engine.

If we use the same output node for both (and other) engines we either
give up on allowing different outputs for the same material. Or we
need to find a solution for this problem.


2) Use the same shader node for Cycles Principled and Eevee Metallic
======================================================
Pros
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* Standard / exchange format

According to Blender Manual the principled node follows an industry
standard model aiming at interchangeability [1].

* Simple for users to learn a single system
* Easy to maintain (code-wise)


Cons
-------
* We may have to choose between performance and accuracy.

UE4 mention in their paper about choices that they made for their PBR
implementation based on Disney's [2]. Eevee shouldn't be slower only
because it would be nice to support Cycles 1:1.

Did I miss any other core point here?

[1] - https://docs.blender.org/manual/es/dev/render/cycles/nodes/types/shaders/principled.html

[2] - https://de45xmedrsdbp.cloudfront.net/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf

Regards,
Dalai
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