Moved handling of the "global" definitions of shaders into main shader generation code.
Added group creation (does not create inputs yet, and remotes should be cleaned).
updated all preview meshes (to fix problems with depth).
Updated all nodes so parameters are located in a "parameters" variable.
Started writing a generic node model whose behavior is defined in a json file. Should be able to replace all generators so far.
Wrote json files for bricks and noise nodes.
Started preparing a custom node.
Exporting a material will now create a SpatialMaterial. The Material graph node
now generates different textures when used as a Godot addon (metallic, roughness
and ambient occlusion are merged into a single texture).
Rendering code (that was in graph_edit and node_base scenes) is now in a single "renderer scene" attached to the main window.
