Margin background color spills into the end of the line in the top and bottom right corners

[knz]

When using MarginBackground, the escape code to terminate the background color is not properly inserted on the first and last line.

See for example:

I would expect the ESC[0m ansi sequence to appear at the end of the first line, instead of the beginning of the 2nd line, like the rest.

Found while working on #109

cc @meowgorithm @muesli

[knz]

Oh and also haha the last margin line at the bottom is not properly colored!