Where to accumulate this renderer's per-frame counters, or null (the
default) to count nothing.
Assigned by whoever owns the record — normally Application, which assigns
it when the application is constructed with statistics on and clears it
again on dispose. Reassigning it mid-frame is undefined; assign it
between frames.
A renderer that reports §84 render counters — the structural capability.
Written as its own interface rather than as a required member of Renderer because adding a required member to a published interface breaks every implementor, and because a backend with nothing to count (a null renderer that draws nothing, an SVG backend whose "draws" are DOM nodes) should be able to say so by omission.