Comment 4 for bug 1378326

So, on this device the 'overlays' are actually 'underlays' in that the fb target layer is always presented as the top-most layer, and the overlays make it to the screen by being alpha-blended with the topmost layer. Mir was clearing this using glClearColor(0.0, 0.0, 0.0, 1.0), and so the fb target layer was blocking the underlays from appearing beneath. We should clear with glClearColor(0.0, 0.0, 0.0, 0.0) in the fallback renderer instead.