layout: Cap each event to 15px of motion

When pressing against the bottom of the screen, we shouldn't
really take more than 15px from each event, to prevent spruious
mouse movements from opening the barrier.

https://bugzilla.gnome.org/show_bug.cgi?id=694467

js/ui/layout.js

@@ -1330,7 +1330,7 @@ const PressureBarrier = new Lang.Class({
 
         this._trimBarrierEvents();
         this._barrierEvents.push(event);
-        this._currentPressure += distance;
+        this._currentPressure += Math.min(15, distance);
 
         if (this._currentPressure >= this._threshold) {
             this.emit('trigger');