In gimp_image_merge_layers() -- the internal function used by the
various layer-merging/flattenning functions -- process the merged-
layer graph in chunks, using gimp_gegl_apply_operation(), instead
of in one go, using gegl_node_blit_buffer().  Processing in chunks
better utilizes the cache, since it reduces the size of
intermediate buffers, reducing the chances of hitting the swap when
merging large images (see, for example, issue #3012.)

Additionally, this allows us to show progress indication.  Have the
relevant gimpimage-merge functions take a GimpProgress, and pass it
down to gimp_image_merge_layers().  Adapt all callers.