image: RegisterFormat is not thread safe

As the title says, image.RegisterFormat is not thread safe.

There was a similar issue regarding database/sql.Register (#9847), where the solution was to protect the slice with a Mutex.

[gopherbot]

Change https://golang.org/cl/118755 mentions this issue: image: make RegisterFormat safe for concurrent use

[bradfitz]

Or we could just document that it's not.

I'm a little surprised that code exists to concurrently register these and it's not just done once early, like from init.

I would say adding a lock would be preferred, as that would mimic the behaviour of database/sql.Register.

[rsc]

I agree that we should make it thread-safe, like we did for database/sql. There are reasonable non-init-time installations, and even at init time there may be other goroutines.