Opacity of last layer is used in postcompose drawing

Reported on the [mailing list](https://groups.google.com/forum/#!topic/ol3-dev/ksBhk80_aqE).

It seems that `context.globalAlpha` set in [ol.renderer.canvas.Layer.prototype.composeFrame](https://github.com/openlayers/ol3/blob/master/src/ol/renderer/canvas/canvaslayerrenderer.js#L48) is not restored after drawing…