* [bitmap] on move, clear old OPAQUE cells #1527
* update ncvisual_blitter_geom() documentation #1547
* replace some ncvisual_blitter_geom with NCVISUAL_OPTION_VERALIGNED
* [normal] replace ncvisual_blitter_geom with NCVISUAL_OPTION_VERALIGNED
* [intro] use ncvisual_blitter_geom() correctly #1547
* ncvisual_blitter_geom: only set *blitter on success
Add `notcurses_check_pixel_support()` and
`ncdirect_check_pixel_support()` per #1367. Removes
NCOPTION_VERIFY_SIXEL, again per #1367. Adds
`free_terminfo_cache()`, and calls it from both
`notcurses_stop_minimal()` and `ncdirect_stop()`.
Update all documentation. Closes#1371 and #1367.
This completes the work for #1068. This addressed a subtle issue.
When we're using pixel->semigraphic art, we want slightly different
rendering. Essentially, imagine that we have two images, each two
pixels tall and one pixel wide. The top image is a transparent pixel
above a white pixel. The bottom image is a white pixel above a black
pixel. We'd expect the result to be two white pixels, but we can
instead get a black pixel above a white pixel. This is because the
*background* color is being merged from the bottom plane, but really
we want the *top* color. Ncvisuals are now blitted along with
information regarding which quadrants they draw over, and when
appropriate, we invert the foreground and background. Closes#1068.
