Actually, Peter (and everyone else) if they have blown glass, they can have flat glass, too -- all you need to do is blow out a large bubble of glass against a metal plate -- the backside of the bubble is flat, and presto! You have a flat plane, as soon as you cut away the rest of the bubble. Of course, you can't make picture windows this way, but you can make individual panes up to a foot or more across.
And of course they have flat mirrors. They just don't use glass for them. Glass? In a mirror? That's a pretty modern idea. They naturally use polished metal mirrors, as has been done since Egyptian times to very near the present. Just take a flat surface of silver, bronze, copper, brass, etc., and scrub it to a high polish, and you end up with a fine mirror. Silver is the best of course -- takes a high polish, and doesn't add a color of its own to the mix, unlike bronze.
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