blitter ought degrade to no more than quadblitter on linux console

[dankamongmen]

Braille and sextants are not present on the Linux console, and are unlikely to be there anytime soon (though see #1333). We ought degrade to NCBLIT_2x2 for both of them (unless NCVISUAL_OPTION_NODEGRADE is in play, of course) when on the Linux console.

acceptance criteria: the plot in notcurses-demo ought generate meaningful glyphs in the linux console. the keller demo still ought break up in the sexblitter/braille portions, since we're refusing degradation there.

[dankamongmen]

if NCOPTION_NO_FONTCHANGES is in play, we probably don't want to go beyond ASCII [shudder].

[dankamongmen]

do we even have degrading in plot blitting? we might not. i think it ought be always done there, if needed (i.e. no option to disable it). though why would this case be that different from media blitting, hrrmmm?

[dankamongmen]

OK. I've added a field braille to struct tinfo. We set this true everywhere but when $TERM equals linux. We degrade from braille to 3x2, and if 3x2 isn't available, we decay to 2x2. We now have a working plotter in notcurses-demo on the linux console.