Be more careful when adding delta

[simoncozens]

Rust has helpful convenience methods for avoiding overflows with arithmetic and avoiding casts; this prevents harfbuzz/rustybuzz#142.

[alerque]

As plausible as it seemed, this isn't quite the right fix: end should never have been usize in the first place. I can submit a PR when the dust settles (although I have some questions about why this fork has a separate codebase that can get out of sync and need this kind of PR instead of being a feature flag on the original port and/or being a PoC branch continuously rebased on it).

[behdad]

Yeah let's keep it close to HB. If overflow happens, that shows a bug in HB code. In this case the fix is indeed to use a signed int.

[behdad]

I have some questions about why this fork has a separate codebase that can get out of sync and need this kind of PR instead of being a feature flag on the original port and/or being a PoC branch continuously rebased on it

I don't think there's a good reason for it, except that the respective owners didn't reach an agreement:

googlefonts/fontations#956