racket/snip: make text snips accomodate height changes via substitution
racket/snip: make text snips accomodate height changes via substitution

Instead of assumning that the snip's text will match a font's height
metrics, check and remember the actual metrics if its doesn't match,
which accomodates font substitions (e.g., when using #\u2144 under
Windows).

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