Normally, we don't bother to edit Sixels in-place upon annihilation, since you can just print the character over them and be done with it. If we move one, however, we need to cut the annihilated pixels out of the Sixel. Failure to do so was leading to flicker around the base of the orca's Segway in the intro demo. This was a bit of a struggle, but it seems to work beautifully (though a bit slower than I would care for -- we'll do something about that). Closes#1555.
Eliminate NCDIRECT_OPTION_NO_READLINE (it was only introduced in
Notcurses 2.1.6). Add a new function, ncdirect_readline(). Upon
first call to this function, initialize libreadline. Destroy
libreadline in ncdirect_stop() iff we initialized it. Add
hilodirect guessing game PoC from #1325. Rename
notcurses_directmode.3 to notcurses_direct.3. Closes#1326.
