Not happy with all these void pointer stuff, but I think this is slightly nicer
to read.

Also not happy with what clang-format has done to my code ;)

I've removed the comment about zoom because it's not clear to me it is still
relevant, but I've preserved the order because, should we have similar stuff
going on, it seems slightly better, and I'm moderately sure the rest of the
callers don't really care.

I think closures coerced to function pointers are nicer (I considered putting
this on a template or using std::function, maybe, this was less code specially
given some callers of EnumerateExternalResources may or may not be hot...),
but if you want me to keep stuff out of line, or move the whole thing to
std::function<> or templates let me know.

Differential Revision: https://phabricator.services.mozilla.com/D40249