A recent fix to discard patch from invalid logical combination
introduced a lot (for large json instance and/or schema) of copies to
make a patch backup. On some case, this introduced up to a 100x slower
validation time.
Resizing the patch object to its previous size avoid unnecessary
temporary allocations from the backup object.
Uses json patch format for defaults: json patch (RFC 6902) makes it
clearer in case of array contents. It is supported by nlohmann::json and
can be applied with `patch` to fill up the json with defaults.