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.
Reclaim the "blitted" and "full foreground" bits in the
channels. Instead, we now write four bits, encoding the
four quadrants we might occupy as the result of a blit.
These four imply the previous two, leaving us with four
free bits remaining in the channels. This opens a clear
path to O(1)-time, zero-space blitter stacking #1068.
w00t!
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.
Don't bias the nccell width by 1, or 0-length EGCs become
255 columns. We weren't actually using the width to drive
much anything until now, so this wasn't a problem, but it
is exposed as an error once we got rid of CELL_WIDE_ASIAN
and start looping through the actual egc column width.
Closes#1278, closes#1277
