pixel of space in between lines in DrRacket.
This change is based on Matthew's experience having a look
at the font setup on the three platforms.
He writes:
> * Mac OS X: the convention seems to be to add space between lines.
> TextEdit, for example, looks like DrRacket: the maze has spaces.
>
> (I can't find a font that makes the maze look right, actually, even
> if I adjust the line spacing.)
>
> * Windows: the convention seems to be that space is built into the
> font. DrRacket (and SirMail) draw lines more sparsely than Notepad.
>
> Perhaps consistent with the differing conventions, the height of
> "Courier New" at 11-pixel size is 14 on Windows, 13 on Mac OS X.
>
> * Unix: the convention seems to be to add space. DrRacket looks like
> the default Terminal and Text Editor programs on Ubuntu.
>
> The maze nevertheless looks right everywhere, because the glyphs
> extend an extra pixel above the declared bounding box!
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