Richard B's patch for resize behaviour. The scrollback now contains

Richard B's patch for resize behaviour. The scrollback now contains
a marker which defines everything before it as `permanent'
scrollback and everything after it as `temporary'; only temporary
scrollback lines are returned to the main screen when the window
height is increased. Screen clears mark the lines pushed into the
scrollback as permanent; so lines explicitly cleared off the screen
by ESC[2J are never returned to it by mistake. This patch also fixes
the incorrect state the primary screen is left in when the window is
resized while the alternate screen is active.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2923 cda61777-01e9-0310-a592-d414129be87e