magnifier: Use all color channels to paint content texture node

In order to paint all the color channels of the content texture we need to
set the color channels to 255, so instead of doing this manually we can just
reuse the static color definition for white.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1020

js/ui/magnifier.js

@@ -65,7 +65,7 @@ var MouseSpriteContent = GObject.registerClass({
         if (!this._texture)
             return;
 
-        let color = new Clutter.Color();
+        let color = Clutter.Color.get_static(Clutter.StaticColor.WHITE);
         let textureNode = new Clutter.TextureNode(this._texture,
                                                   color,
                                                   Clutter.ScalingFilter.NEAREST,