PowerPoint fits shrink-on-overflow text again only when the text changes,
never when a file opens: it draws the saved fontScale/lnSpcReduction even
where our metrics would call the text overflowing (trailing empty
paragraphs at full size, substituted fonts). Shrinking further on open made
saved decks text smaller than PowerPoints (74 % instead of 85 % on a real
deck). With no saved scale, or once an edit has reset it, the text is
fitted as before.