vellum memoises compiled scenes keyed on an object-identity token so repeat
renders of an unchanged scene (multi-format export, a display() resize back
to a prior size, or animation replaying a fixed set of frames) are cheap. The
cache is transparent — a cached render is byte-identical to an uncached one —
and bounded (options(vellum.cache_size=), default 8), so you rarely need
this; it is provided to reclaim memory or to force a cold render in
benchmarks. Disable caching entirely with options(vellum.cache = FALSE).