VNCSConnectionST: Release mouse button(s) on close

VNCSConnectionST: Release mouse button(s) on close

When the connection is severed, release all mouse buttons.

This fixes an issue with x0vncserver where the local display
would stop responding to mouse events if the connection closes
while the remote user was holding down a mouse button.

To reproduce, start a VNC client, press and hold a mouse button
and then kill the connection (e.g. close client, kill server,
yank network cable). This caused the local screen to no longer
respond to any mouse-down events until a VNC client reconnects
and clicks anywhere.

This may inject a "mouse release" event while closing, however,
if you click on your screen and then yank the mouse cable, a mouse
release event also would not come unexpectedly, so the cure's
side effects aren't as bad as the disease.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>