Increase quality of the thumbs in the Asset Library

Current preview render is fast but renders only diffuse with some specular. Sometimes it's OK and sometimes (in my case mostly) it's clearly not enough, especially when blender has dark interface. I propose to add the possibility of generating automatic pbr-quality preview for the assets. It can be done in two most obvious ways:

1. Built-in scene that is used to render the asset's preview in background when the Generate Preview button pressed
2. Custom scene that can be configured by user himself

Also being able to activate and deactivate transparency could help in some cases.

And yes, I know that we can add our custom previews, but when you have a lot of objects, it's better to be automated.