Two more small bugs introduced by the B-tree reorg: scroll() now

requires fix_cpos() to be called after it (otherwise cpos might point
to a line that isn't where you remember it being), and a mis-aimed
incpos() was causing forward selection dragging not to include the
char under the mouse. Both fixed.

git-svn-id: svn://svn.tartarus.org/sgt/putty@1063 cda61777-01e9-0310-a592-d414129be87e