selectArea: Ignore motion events once we got a result

When selecting an area for screenshot we monitor the events while we've valid
coordinates in order to redraw the rubber band.
However, we don't stop ignore the motion events after button release and so
while animating. This might cause an unwanted effect if moving the mouse away
during fade out that is way more visible slowing-down the animations.

To fix this ignore any motion event once we've set the results.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/711

js/ui/screenshot.js

@@ -271,7 +271,7 @@ var SelectArea = class {
     }
 
     _onMotionEvent(actor, event) {
-        if (this._startX == -1 || this._startY == -1)
+        if (this._startX == -1 || this._startY == -1 || this._result)
             return Clutter.EVENT_PROPAGATE;
 
         [this._lastX, this._lastY] = event.get_coords();