sixel support added

[nick black]

i've been hard at work over this weekend completing the
NCBLIT_PIXEL interface, which will support Sixel, Kitty's
graphics protocol, and any other straight pixel language.
i'm pleased with how i've integrated pixel graphics -- each
(atomic) sixel image is contained in a single nccell, stored in
the egcpool in its entirety, encapsulated by the command
sequences to enter and exit pixel mode. we follow a sixelimage
output with a hard cursor reposition, etc., things are good.

at 08:16:02 EST this morning, we drew our first complex sixel
image (see attached).

plenty of libraries generate sixel (i don't know any portable to
kitty+sixel, but they might exist). plenty of libraries let you
do TUI crap. i know of no other ones which allow you to sanely
integrate the two, which is what i'm aiming for. i.e. you'll be
able to slide them around, place them below or above text
elements on other planes, and possibly even blend them.

it took some time, but pixels ahoy! thanks to José for poking me
on this, and especially pointing out the recent Alacritty work.

--
nick black -=- https://www.nick-black.com
to make an apple pie from scratch,
you need first invent a universe.