ncplane_as_rgba: genericize across blitters
ncplane_as_rgba() previously was implemented explicitly
in terms of NCBLIT_2x1. it now uses the `egcs` field of the
`blitset`. this supports NCBLIT_2x2 and NCBLIT_3x2 in
ncplane_rgba(). Closes#1490.
* Paint sprixels in order, bottom-to-top
We don't want to have to track sprixel order whenever someone
moves an ncplane, so just keep a list growing backwards as we
pass top-to-bottom in notcurses_render_internal(). Each time
we hit a sprixel plane, splice it out of the sprixel list, and
add it to the front of our temporary list. When we hit the
bottom, stick this temporary list on the end of our existing
list (any such planes are to be deleted, which comes before
drawing). Closes#1575.
* reorder collected sprixellist; solves kitty but breaks sixel =/ #1575
* remove debugging cruft
* [rust] fix up mergedown mutability
* [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
deprecate ncvisual_geom() in favor of new ncvisual_blitter_geom(), which allows caller to get the blitter used. replace the checks and calculations in ncvisual_render() with a call to ncvisual_blitter_geom(), unifying the two paths (and eliminating several bugs in the unloved ncvisual_geom()).
Deprecate ncplane_align(), after rewriting it as a passthrough
to new function ncplane_halign(). Add ncplane_valign(). Update
all callers. Closes#1468.
