tsm: screen: fix cleaning buffers on resize

tsm: screen: fix cleaning buffers on resize

There is some nasty bug where we do not correctly clean buffers when
resizing. So we now clear the whole offscreen region on resize to go sure
everything is clean.

Note that cells can pretty much move everywhere during their
offscreen-life so this is a quite sophisticated task to keep track which
cells are clean and which not. Therefore, simply clean all of them when
taking them on-screen.

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