[skrifa] autohint: cycle detection for GSUB lookups

[dfrg]

Adds a stack to detect cycles in GSUB lookups while computing shaper coverage for autohinting.

Fixes #1295

[rsheeter]

I don't love that this makes it relatively easy to add code that enters without calling exit. This would be somewhat harder if it was directly in the process lookup fn? - depicted below. Or by a guard that reduces depth on drop but I suppose that path takes you towards multiple mut refs.

    fn process_lookup(&mut self, lookup_index: u16) {
        // Guard: don't cycle and don't go exceed depth limit
        // Note: we use a linear search here under the assumption that
        // most fonts have shallow contextual lookup chains
        if self.lookup_depth != 0
            && (self.lookup_depth == MAX_NESTING_DEPTH
                || self.lookup_stack[..self.lookup_depth].contains(&lookup_index))
        {
            return;
        }
        self.lookup_stack[self.lookup_depth] = lookup_index;
        self.lookup_depth += 1;

        // Actually process the lookup
        self.process_lookup_inner(lookup_index);

        // Out we go again
        self.lookup_depth -= 1;
    }

[dfrg]

I agree with the sentiment but I kept the stack ops separate to support isolated unit testing of that functionality. If we embed them then we need to build a lookup list with the desired structure which seems less clear and more fragile to me.

[rsheeter]

I would have thought we need to build lookups with good/bad structure (one that cycles, one that has max depth, one that has max depth + 1, etc) to unit test regardless so... that's kind of fine?

[rsheeter]

might as well mark inline always?

[dfrg]

sgtm

[dfrg]

For reference, contextual lookup depth in all of Google Fonts and Noto (10604 font files):

depth	# top level lookups
2	76338
3	933
4	14
5	14
6	42