Operating system or device, Godot version, GPU Model and driver (if graphics related): Windows 10, Godot Git 5614902, NVIDIA GeForce GTX 1080

Issue description: The clearcoat effect on materials is too subtle, and isn't noticeable at all on most materials of the Material Testers demo; it's only very slightly noticeable on the "mirror" material. This may hint at a bug in the PBR shader.

Steps to reproduce: Create a SpatialMaterial, enable its Clearcoat property, then play around with the Clearcoat and Gloss values.