A growable pool of reusable Ts — lets a frame-producing element
(see crate::elements::SwDecoder, crate::elements::Scaler) hand
out the same buffers over and over instead of allocating a fresh one
(and freeing the old one) every single frame. Reusing an
already-allocated ffmpeg_next::frame::Video also lets ffmpeg’s own
avcodec_receive_frame/sws_scale skip their internal buffer
allocation when the reused frame already matches — not just savings
on this crate’s side.
One item borrowed from an UnboundObjectPool — Deref/DerefMut
to T for normal use, and returns itself to the pool (after running
release on it) when dropped.