pointerA11yTimeout: Fade the pie timer in

Fade the pie timer in using a duration of 1/4 of the timeout and a
EASE_IN_QUAD animation. This significantly reduces flickering of the pie
timer while moving the cursor and makes the timer less distracting.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/688

js/ui/pointerA11yTimeout.js

@@ -69,6 +69,7 @@ var PieTimer = GObject.registerClass({
     start(x, y, duration) {
         this.remove_all_transitions();
 
+        this.opacity = 0;
         this.x = x - this.width / 2;
         this.y = y - this.height / 2;
         this._angle = 0;
@@ -76,6 +77,12 @@ var PieTimer = GObject.registerClass({
         this.show();
         Main.uiGroup.set_child_above_sibling(this, null);
 
+        this.ease({
+            opacity: 255,
+            duration: duration / 4,
+            mode: Clutter.AnimationMode.EASE_IN_QUAD
+        });
+
         this.ease_property('angle', 2 * Math.PI, {
             duration,
             mode: Clutter.AnimationMode.LINEAR,