performance(pty): update last render time on non-backed-up render

[imsnif]

This includes two minor fixes following that changes in #1585

1. We now make sure that when the screen thread is not backed up (i.e. there's room in its buffer to receive immediate render instructions rather than buffering them), we update the render deadline so that timed-out renders will be sent properly.
2. That being said, we also ignore the render deadline if the screen is not backed up to prevent extraneous renders