This is done by allocating cells for the pre-edit text when receiving
the text-input::done() call, and populating them by converting the
utf-8 formatted pre-edit text to wchars.
We also convert the pre-edit cursor position to cell positions (it can
cover multiple cells).
When rendering, we simply render the pre-edit cells on-top off the
regular grid. While doing so, we also mark the underlying, “real”,
cells as dirty, to ensure they are re-rendered when the pre-edit text
is modified or removed.
