Looking through all my work notes, it seems that all the
problematic aspects are now solved in a satisfactory way.
* it is clear how to dependency-inject a `BufferProvider`
* the internals of the `BufferProvider` front-End have been restructured
* there are now two ''pillars'' of the implementation: `BufferStage` and `BufferStore`
* both can be subclassed independently
* `BufferProviderSetup` exposes a proxying-mechanism (used for `DiagnosticBufferProvider`)
* the `BufferMetadata` have been reviewed and some minor details were rectified
* after a re-Design of `OutputSlot`, `BuffHandle` can now be used as unified front-End
* both `DiagnosticBufferProvider` and `DiagnosticOutputSlot` have been rewritten
* they are now independent, self-contained specialisations, layered on top
Overall, what might seem like a detour, was an important step towards
well-balanced structures and a better understanding of the Render Engine.
