Do not run cleanup while printing is ongoing.

During investigation of #4994 it became clear that one possible problem with fonts in print is that while printing is ongoing a font is removed and reloaded. If cleanup was set to a low number the problem disappeared.

the cleanup was scheduled when no page or thumbnail was rendered. Because printing uses a different mechanism, t could happen that printing occurs at the same time as cleanup. This is consistent with the observation that the error was reproducible after a few tries (most likely just hitting the 30 second mark).

The solution is to not run cleanup while printing is ongoing. Therefor a flag is set and renderHighestPriority() invoked before and after printing.
Shouldn't this block be moved into https://github.com/mozilla/pdf.js/blob/master/web/viewer.js#L1264 instead?
As it currently is, there doesn't seem to be anything preventing an already set `idleTimeout` from triggering (thus running `cleanup`) while printing is happening.