add option to disable autoscrolling

[diggy]

Hi all and kudos for a great product to use.

My request is simple.
Because I use the viewer extensively with x11vnc and ncache, that caches parts of the display below the current one, auto scrolling when near the bottom of the window makes use a pain in the neck.
I think the quick approach would be to have a toggle button on the toolbar that turns on/off auto scrolling and scrollbars and it's status gets saved in the vnc file per session.

A second better approach would be to add an option to crop the pixels sent from vnc to a user defined size so as to keep the cache invisible to the user. That way the user could use auto scroll if needed of even use autoscale (when using ncache now gives a x*z screen where z = ncachesize * the user resolution)

That is all..

Thank you in advance.