When creating a flatten node, which is used when removing alpha
channels and when flattening an image, use a gimp:normal node to
combine the layer with the background color, instead of a gegl:over
node.  gegl:over can apparently result in completely black output
with OpenCL enabled, under certain (not fully pinned-down)
conditions.

As long as the OpenCL version of gegl:over is borked, there is not
much reason to use it over gimp:normal, which is more consistent
(in intension, if not in extension) with the rest of the
compositing pipeline.