This was an unused codepath and it complicates some things I'd like to
do, such as resetting our pools during resize. It complicates those
paths because if a user provides a pool we can't reset it (because other
things might be in it). It's best to own the pools. And since we didn't
reuse pools anyway, let's remove that.
Note this was previously used by our old render state mechanism (before
`terminal.RenderState`) as a way for the renderer to speed up clones by
having a preheated pool that was likely the right size before every
frame. Since we changed methods, we don't need it.
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