v2.2.3—ambitionz az a ridah

Rather more of a "technology preview" release than I'd like, but big things were building up, and needed a release:

• EXPERIMENTAL support for pixel graphics via NCBLIT_PIXEL. They pretty much work, but they're neither as fast nor as attractive as they'll eventually be, and they interact poorly with intersecting planes. As standalone units, they're not too bad. See #1380, #1383, #1381, #1378, #1373, and #200. ncplayer and ncls can both now use pixel on terminals supporting Sixel with -bpixel. ncneofetch will use pixels where supported. Pixel graphics will be shown off in the keller demo when supported.
• INITIAL support for the nctree widget. There will be some more work involving expanding and collapsing, but this gives a pretty accurate portrait of where it's going. See #1164. New tree PoC.
• Work around annoying export of bswap in recent OpenImageIO.
• More robust output (always loop on EBUSY et al), including a fix for the crafty #1090.
• Treat SIGCONT like SIGWINCH, for programs emerging from backgrounding.
• Improve transparency in zoo demo (#1353).
• Improve quadblitter accuracy (#1354).
• Restore prior signal handlers in drop_signals(), catch SIGILL as fatal (#1357).