Detect non-monospaced X fonts, and respond by drawing text one

Detect non-monospaced X fonts, and respond by drawing text one
character at a time centred in its character cell, as we do for
Pango. Gives much better results for those non-monospaced fonts
which are usable as terminal fonts, and shows up the problems with
the others more readily. (In particular, this means the preview pane
in the font selector now warns you there will be trouble if you
select such a font.)

[originally from svn r7949]