Stop sending release events for mouse wheel 'buttons' in X mouse mode.

Stop sending release events for mouse wheel 'buttons' in X mouse mode.

On Windows (X mouse reporting of the mouse wheel isn't currently done
by the Unix front end, though I'm shortly about to fix that too) a
mouse wheel event is translated into a virtual button, and we send
both a press and a release of that button to terminal.c, which encodes
both in X mouse reporting escape sequences and passes them on to the
server. This isn't consistent with what xterm does - scroll-wheel
events are encoded _like_ button presses, but differ semantically in
that they don't have matching releases. So we're updating to match
xterm.

git-svn-id: http://svn.tartarus.org/sgt/putty@10138 cda61777-01e9-0310-a592-d414129be87e