Don't search beyond Sync roots for highest priority work

[gaearon]

Fixes #13334 at a cost of one extra comparison per scheduled root.
Not directly observable — verified by manual testing with test cases from #13334 and #12700.

Unless I'm missing something it's useless to continue searching once we've found sync work since nothing else is going to win.

[sebmarkbage]

I'm convinced we should fix the general issue though since even just always picking the first one is still inline with semantics and even better for a single root.

[gaearon]

By "fix the general issue" you mean "stop prioritizing across roots" or...?

[sophiebits]

Why is this n^2 in the first place though?

[gaearon]

We schedule N roots in a lifecycle.
Then we:

1. Search for highest priority root
   • This is naïve and always walks all of them
2. Render it
3. If there are more scheduled roots, repeat

[gaearon]

Regardless of how we can optimize it, it seems like the current search procedure is useless after encountering a Sync root so I'll get this in.