[Fontconfig] Marking glyphs as deliberately blank, per font

[Paul Flo Williams]

I have been attempting to create some fonts that reproduce the look of old
text terminals. One of the features of terminals like the VT100 was
double-height, double-width mode on certain lines. In order to do this, I
have created two fonts, one for the upper half of characters, and one for
the bottom half. However, for characters where all the marks would appear
in just one half of the glyph, say quote marks or the underscore, I am
left with blank glyphs in places where fontconfig doesn't expect them, and
those glyphs are marked as broken by fontconfig, and substituted.