and pass them where we know the buffer could be from another image.
Pass "NULL, 0" if we know it's the same image.

Add gimp_layer_new_convert_profile() which takes the newly created
layer and the ICC data/length; call it from both
gimp_layer_new_from_gegl_buffer() and gimp_layer_new_from_pixbuf().

gimp_layer_new_convert_profile() is empty, this is just
infrastructure.