font: have Glyph return !ok for U+FFFD substitute

The other return values may still be non-zero, but this lets callers
identify when substitution happens.

"TODO: is falling back on the U+FFFD glyph the responsibility of the
Drawer or the Face?" was resolved. The answer is "the Face". For
kerning, the previous rune is unchanged (and not set to U+FFFD).

This also fixes an inconsistency in the basicfont.Face implementation,
where GlyphAdvance and GlyphBounds would unconditionally return a
non-zero advance, but Glyph could return a zero advance when the Face
doesn't have a U+FFFD entry.

Fixes golang/go#58252

Change-Id: Ie97e68e1d5e2efd13c9e84ad12db4495d83a5ca3
Reviewed-on: https://go-review.googlesource.com/c/image/+/474376
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Nigel Tao (INACTIVE; USE @golang.org INSTEAD) <nigeltao@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>