tsm: screen: fix including final character in selection

tsm: screen: fix including final character in selection

We need to draw the final character of a selection with inversed
background, too. This is a bit tricky as the selection may be inversed
itself. Therefore, we just keep a flag that tells us whether the previous
character was selected and just draw the new character also selected.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>