Background colors aren't working properly

[dankamongmen]

When I run notcurses-demo, I'm seeing two problems:

• The background color for the first screen seems to be chosen almost at random
• The background color for the widecolor demo is the same for the entire page

[dankamongmen]

Background colors are working properly in the griddemo tests. In the wittgenstein demo, we get it working properly on the one multibyte character, but not the others...