Support font fallback even when an X11 server-side font is selected,

Support font fallback even when an X11 server-side font is selected,
by introducing a wrapper around an individual unifont which falls back
to Pango (which already has built-in fallback) in the case where the
selected font doesn't support the glyph in question.

The wrapper itself is a (vestigial) subclass of unifont, to minimise
disturbance at the call sites.

[originally from svn r9293]