Add preset material, pull more variables into the template
The BitsquidPBR.blend file includes a preset material which is set up to mimic the "core/stingray_renderer/shader_import/standard" shader graph. This single material is loaded into the current project through Blenders "Append..." API when a file is loaded (or a new one created).
It also pulls the basic values into the material template. The map textures can be added later.
There is also a small fix for the `MATERIAL_OT_bitsquid_export`, because it didn't seem to work on my system (Blender 2.92.0).
See merge request lschwiderski/bitsquid-blender-tools!1
