v2.4.3—the boss like king koopa

• Use posix_spawn() and its ass-ugly API in place of fork()+exec*() #2194.
• Fix a bug in ncplane_erase_region(), and make it much more general. It can now be used for cursor-relative erasure through the end/beginning of the current line, etc #2181.
• It was recently discovered that many terminals, including XTerm, had inverted the semantics of DECSDM aka "Sixel Display Mode". Since then, a number of these terminals have changed their semantics to match the VT340 semantics. Unfortunately, this means older versions of Notcurses now display all sixels in the upper left of the visual area. Notcurses now knows the semantics changed in XTerm 369, the ayosec/graphics branch of Alacritty 15.1, foot 1.8.2, and MinTTY 3.5.2 #2205. Many thanks to @j4james for his assistance here, and general spearheading of this ecosystem-wide change.

If it weren't for that last, I probably wouldn't have bothered to release just yet. Most work this week was on #2183, which isn't easily visible from the outside, but is absolutely critical for robust and performant handling of input. It's been a fascinating problem -- take a look at #2208 to see what all's going down. automata theory, huzzah! but yeah, not too much here, been busy at the day job, sorry i'll try to suck less ass.

btw if anyone needs multi-gigabit low-latency satellite transit, i can get you a good price!