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.
