rdp: fix "window size" resolution

rdp: when "window size" resolution is selected, use the actual available window size, not a cached size. Currently when using "window size" resolution you get some annoying scrollbars in the remoteview, because it uses a resolution cached in preferences at save time, determined from the mainwindow size (including the menubar, tabbar, etc..). This PR recalculates the available size on the window just before starting the session, so that the remove video fits in the window perfectly: no scrollbars, no need to scale.