Fix the problem caused by wrong cursor coordinates. No more messing with
the main cursor, instead the cursor is saved when receiving a pre-edit
text update and used for drawing later.
Update the overlay to the last visible cursor position before rendering
to ensure it always moves with the cursor. Finally, draw the overlay
after line rendering is complete, and restore the line buffer after
updating the rendered data to ensure that the line text being read is
correct at all times.
This also improves performance by only rendering once when changes are
made, eliminating the need to repeatedly disable and draw after various
commands and not even comprehensively.
126aaddccb