Make scripts without standard characters actually work.
This also enables `--fallback-stem-width' for non-symbol fonts, as
documented.
Problem reported by Pablo Moleri <PMoleri@infragistics.com> – the font in
question (Hind Guntur Regular v1.001) missed standard characters in the
fallback font, which produced buggy bytecode, causing really bad artifacts.
I can no longer remember why I disabled hinting for scripts without standard
characters (in commit
7e4dd79d), which contradicts the documentation...