We failed to convert the byte-indices to cell indices, resulting in a
box cursor covering the entire pre-edit string.
Note that in addition to fixing the translation from byte index to
cell index, the rendered had to be updated to dirty one extra cell
from the original grid.
Without this, we left trailing cursors behind us when the user deleted
text from the pre-edit string.
We may want to be able to enable/disable IME run-time, even though we
have received an ‘enter’ IME event.
This enables us to do that.
Also add functions to enable/disable IME on a per-terminal instance
basis.
A terminal may have multiple seats focusing it, and enabling/disabling
IME in a terminal instance enables/disables IME on all those seats.
Finally, the code to enable IME is simplified; the *only* surface that
can ever receive ‘enter’ IME events is the main grid. All other
surfaces are sub-surfaces, without their own keyboard focus.
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.
