Avoid reusing the custom data type enum with additional values. Instead
use std::variant and type names to properly distinguish between custom
and generic attribute requests. Use a Vector to hold the requests.
Also attempt to simplify the string key building process for requests
and groups of requests in batches. Previously for every PBVH node it
would rebuild the key 3 times, now it only does it once. It's hard to
measure, but that process did show up in profiles, so performance is
probably slightly improved when many nodes are handled at once.
Hans Goudey
8c8ea2ec47