Fix a bug in scroll_display(): when the scrolled region contains the cursor

Fix a bug in scroll_display(): when the scrolled region contains the cursor
we need to update dispcurs as well as disptext.

Add scroll optimisation to scroll().  This makes it rather obvious that scroll
optimisation is breaking with PuTTY's usual policy of lazy updates, since
scrolling is done eagerly.  Fixing this so that all the scrolling is saved
up for do_paint would be nice, but non-trivial.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2252 cda61777-01e9-0310-a592-d414129be87e