v2.2.10—reminiscent of a whale

So I'd somehow gotten into my head that the variable is BUILD_TYPE, not CMAKE_BUILD_TYPE, and so I've been setting the former to Debug for...who knows how long, assuming that was testing with all my assert()s, which obviously it was not. Thus we get embarrassing assert()s firing all over notcurses-demo (see #1630 and #1629). Those have been addressed; thus this brown-bagger of a release. Sorry about that.

We also got #1625 handled with a big assist from @dnkl , so ncdirect_cursor_down() will scroll as (presumably) expected when on the last line. This fixes the dirlines PoC; thanks to @joseluis to noticing the problem and filing a bug.

The new NCVISUAL_OPTION_CHILDPLANE allows you to indicate that the n field of ncvisual_options refers not to the plane onto which you'd like to blit, but the parent of a new plane. A plane will be created using the other parameters in the ncvisual_options, as a child of this parent. This means things like, say, vertically centering a sprixel relative to the standard plane can be done in one step (#1603).

The new wipebitmap PoC identified two problems, both of which have been rectified (#1631). Kitty bitmaps could be rebuilt using slightly incorrect alpha values, and rebuilt bitmaps weren't always invalidated as necessary for redisplay.