Add a new member 'sextants' to the terminfo cache (both
notcurses and ncvisual contain one of these, and both
initialize it the same way -- interrogate_terminfo()).
Add a new function, 'notcurses_media_defblitter()', and
deprecate 'ncvisual_default_blitter()' (the latter didn't
receive enough information to return NCBLIT_3x2). Update
all callers. Add new *internal* function rgba_default_blitter(),
so this logic can be freely changed in the future. If
sextants are available, and we're scaling, return NCBLIT_3x2.
Once we detect sextant availability, we'll have sexblitter
as a default -- stay tuned! #1114
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