screenshot: Extend ScreenshotArea parameter validation

We currently only ensure that width and height are positive, so it
is still possible to pass in values that don't make any sense at all
(which may even result in a crash when exceeding limits imposed by
X11).
There is nothing to screenshot outside the actual screen area, so
restrict the parameters to that.

https://bugzilla.gnome.org/show_bug.cgi?id=699752
screencast: Fix disabling screencasts via session mode

If screencasts are disabled, we return a DBus error, but still start
the recording happily - add early returns in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=699752
screencast: Validate parameters of ScreencastArea

... just as we do for screenshots.

https://bugzilla.gnome.org/show_bug.cgi?id=699752