Missing characters in monospace fonts fall back to proportional fonts

I tried a couple different fonts. Some have the problem, and others do not.

A sample of fonts that have the problem:
* Droid Sans Mono
* TlwgMono Medium
* Ubuntu Mono
* Terminus

A sample of fonts that do _not_ have the problem:
* Andale Mono
* DejaVu Sans Mono
* FreeMono

Yes this is clearly visible if you try to run, for example, Dwarf Fortress in console mode.
For example the smileys used for the dwarves themselves ☺ are almost two characters wide in gnome-terminal with the default font.

Status changed to 'Confirmed' because the bug affects multiple users.

In a terminal, enter:

    pango-view --font="Ubuntu Mono 48" --markup --text='<span fallback="false">ABC&#13;A☺C&#13;A▷C</span>'

This demonstrates that:
- the ☺ character in Ubuntu Mono is the correct width
- the ▷ character is not present in Ubuntu Mono at all.

The lists of fonts that "have the problem, and ... do not" seem to be lists of fonts that contain those two characters or not.

So the bug here probably is not with any individual font; it is that monospace fonts fall back to proportional fonts for missing characters, instead of falling back to another monospace font with the desired glyph.

Now I think about it more, even just ensuring that monospace fonts fall back to other monospace fonts wouldn't solve the problem, for two reasons. First, monospace fonts can be different widths, so the fallback might be wider than the original. And second, the character might not be present in any monospace fonts at all, but still be present in a proportional font, and it would be better to fall back to a proportional font than to nothing at all.

So ideal behavior, perhaps, would be ensuring that a fallback character for a monospace font is the same width as the original font -- by centering it if it is narrower, and by scaling it down if it is wider.