Fix bug #14170 with window-end returning a stale value.

Fix bug #14170 with window-end returning a stale value.

This bug was introduced in 2012-10-15T09:03:56Z!rudalics@gmx.at
while solving bug #12600.

 src/window.c (Fwindow_end): Test more flags, including the buffer's
 last_overlay_modified flag, to determine whether the window's
 display is really up-to-date.  Prevents the function from
 returning a stale value.
 (Fwindow_line_height): Fix the test for up-to-date-ness of the
 current matrix.