Add more useful 3D viewport rendering modes

Created by: Calinou

Now that Godot includes a wireframe mode for the 3D viewport, more viewport rendering modes could be added for the convenience of developers and artists alike.

Outline mode

Outline mode example

This mode displays materials as usual, but displays a wireframe on top of them. The color should be settable by the user, but probably should default to a white or light gray color.

Example use case: Displaying geometry complexity while still being able to freelook through a level easily.

Lighting mode

Lighting mode example

This mode sets the normal map strength of all materials to 0, sets the albedo texture and color to pure white (#ffffff), disables specularity, sets roughness to 1 and metallic to 0.

Example use case: Analyzing lighting conditions in a level, regardless of the applied materials.

Footnote: the screenshots come from Cube 2: Sauerbraten, I am not working on an implementation of these modes in Godot.