This basic implementation simply upscales the viewport if needed.
This ensures crisp display regardless of the OS, and ensures
OS-specific fallbacks don't interfere with the program.
A proper hiDPI implementation will be needed to have better-looking
fonts and rendering.
This partially addresses #8.
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).
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.
