Optimize typesetter mode handling

[alerque]

This is awkward. I was working on something else and thought I spotted an easy way to optimize the typesetter look and save quite a bit of time. I threw it in and kept working on my other thing intenting to come back and benchmark it later.

It turns out this broke the world. Some tests started generating infinite pages and building the manual started hanging completely.

This means that typesetter:leaveHmode() is doing work even when already in vertical mode. I would have expected all the node queue processing to be a noop.

This is almost certainly intertwined with other typesetter:pushBack() issues that have surfaced, but perhaps the fact that this function is doing work outside of its labeled scope is a clue where the algorythm is breaking down.

[Omikhleia]

Yep, I had seen this too: leaveHmode is also where the page building logic is triggered (and even if the horizontal queue is empty, we might have additional things in the vertical queue: penalties, vglues etc.)