initial opacity setting sticks

[ahocevar]

Caused by  http://trac.osgeo.org/openlayers/ticket/3431#comment:7, the following fails:

1. Open http://openlayers.org/dev/examples/lite.html;
2. On the development console, do the following:

   var layer2 = layer.clone();
   layer2.opacity = 0.5;
   map.addLayer(layer2);
   map.removeLayer(layer);
   layer2.setOpacity(1);
   

The opacity will still be 0.5, because it was set on every tile of layer2.

This will be fixed with #3419.

[ahocevar]

closing, because the r12185 which caused this was reverted.