This is enabled with bl_use_eevee_viewport = True. It allows external engines to generate an Cycles/Eevee shader node setup to emulate their materials in the realtime viewport, or to leave it to users to do manually. Removed bl_use_exclude_layers and bl_use_shading_nodes that did nothing anymore. This should not break API compatibility, any scripts setting those should continue to work the same as before. Also adds descriptions for some RenderEngine settings.