In the end I refrained from applying most of the changes I had intended.
I am aware that `BufferProvider` is still quite a massive structure,
yet with the implementation refactorings done thus far, it seems
to be reasonably clear and self-contained now.
It would be possible to totally overthrow the grown structure and
expose only the two handles (BuffDescr and BuffHandle), together with
a factory function, but I cannot see how this would help to make the
code clearer and more approachable. Maybe it would make the usage
more obvious from the client's perspective — yet, to be honest,
there are not ''that much clients'' to justify such a move.
Furthermore, the implementation is far from finished: for both of
the back-end lines, a fully production-strength implementation
must be created, that is able to operate in a highly concurrent
environment; very likely some lock-free messaging and hand-over
between threads will become necessary, adding to the complexity.
Thus it seems better to note the ''design critique'', but to
postpone any further refactorings to a point where we can watch
a fully developed Render Engine under production load.