Current redrawing is buggy (see several bugs in here), and also flickers.

It should be rewritten so that all redrawing is done from the expose_event
handler of ZvtTerm. That is, all code outside of expose_event should simply 
call gdk_window_invalidate_rect() to invalidate a region of the terminal 
window. In expose_event, the code should determine which terminal lines 
intersect the exposed area (event->area), and redraw those lines.
_Only_ the expose handler should ever draw anything.