bug#20499: [PROPOSED PATCH] C-x 8 shorthands for curved quotes, Euro, etc.

[Drew Adams]

> > We could set up a way to test whether a code point can be
> > displayed, and skip scripts that can't be displayed.
> 
> Alas, we don't know which cannot be displayed until we've tried and
> failed.

Where is this try-and-fail done?  Is it only in C code, or is
there some Lisp function (predicate) that you can call to tell
you whether a given char can be displayed in a given (e.g. the
current) font.

Even if such a predicate would need to try displaying, to find
out whether it is possible, this could be useful.

It would be good if we could, for example, optionally show only
chars that the current font can display.