make synchronized rendering configurable

[christianparpart]

that is, while it is great to have, it might make sense, sometimes, to have it disabled, e.g. when trying to trace through each VT sequence interactively, and really wanting to see what is happening on the screen.

This is a use-case exclusively to those, that have a deep understanding of VT sequences. It however certainly helps debugging VT sequences (or trying to understand the use of such).

Configuration could happen through configuration file (per profile).