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
ncplane_destroy() needs to call ncplane_reparent_family(), not
ncplane_reparent() as it was doing (closes#1291). ->absy and
->absx actually are absolute; return them directly for an O(1)
ncplane_abs_yx() (down from O(N), huzzah). Add some unit tests
related to destroying and reparenting (#1286). Add ncplane_abs_y()
and ncplane_abs_x(), document them, etc.
Improve the documentation for ncplane_set_base_cell() and
ncplane_set_base(). Go by render.c's authority (all three
channels are independently checked for transparency, and
replaced with the base cell if so--egc, fg, and bg). Add a
BASE CELL section to notcurses_plane.3. Check for, and reject,
an nccell which is secondary among a multicolumn EGC.
Closes#1238.
