iOS 10: cicontext.render() makes transparent areas black : <p>I'm using</p>

<pre><code>ciContext.render(overlayImg, to: pixelBuffer, bounds: bufferFrame, colorSpace: nil)
</code></pre>

<p>where <code>overlayImg</code> is an image with transparent areas. Pre-iOS10 this worked as expected, but since iOS 10 the transparent areas are black.</p>

<p>I don't know exactly what changed, but how can I achieve the rendering with transparency again?</p>
