Fix bug in redisplay: when variables that affect display (e.g.

Fix bug in redisplay: when variables that affect display (e.g.
truncate-lines) are changed, the window start marks must be erased.
Otherwise, the old start marks are used, and with the new display
variables in effect, these marks could cause point to be left
offscreen, or even force all of the buffer's text to be offscreen!