with proper value names. Mark most values as _BROKEN because they use
weird alpha compositing that has to die. Move GimpLayerModeEffects to
libgimpbase, deprecate it, and set it as compat enum for GimpLayerMode.
Add the GimpLayerModeEffects values as compat constants to script-fu
and pygimp.
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.
