Add brushes as resources to the scene manager

As it turns out, creating brushes is actually quite expensive with a lot of rendering backends. So we definitely want to only create them once and then just attach them to the individual entities.

Start by tracing the scene manager's brush creation path and how brushes are attached to individual entities. Check the rendering-backend integration points to understand the existing resource lifecycle. Done means brushes are created once as scene-manager resources and reused by entities without changing rendering behavior.

Written by the indexing model from the issue text.