Audit for code that can lead to excessive runtime with malicious/malformed data

[dfrg]

Specifically cases where we loop based on untrusted counts and/or iterate over ranges where the the sizes aren't bounded by data and thus caught during parsing.

For example, the iterator for cmap format 12 can be forced to do 2^32 iterations n times where cost is only n * 12 bytes of malicious data.

[rsheeter]

@behdad does HB protect against this? Have a budget for instance?

[behdad]

Yes we have a budgets for a lot of operations. For cmap I think we skip anything outside valid Unicode range.

[rsheeter]

To create the xref, #954 fixes this for cmap12.