Add support for separated 4-cell mosaic graphics

Add support for separated 4-cell mosaic graphics

The shapes of the 4-cell mosaic graphics are based on an alternative
history in which Teletext adopted 4-cell graphics.  It's obviously
reasonable that such a world would still have separated graphics, so
let's have them.

This makes me thing that the method I've used for flagging separated
graphics is silly, but that's a story for another commit.