Don't call ReleaseCapture() on any mouse-button-up event. Instead,

only call it when the _last_ mouse button comes back up. Otherwise,
xterm mouse tracking will lose a button-up event if you press down
two buttons, move the mouse outside the window, then release them
one at a time.

git-svn-id: svn://svn.tartarus.org/sgt/putty@8425 cda61777-01e9-0310-a592-d414129be87e