Add a composite_space parameter to gimp_gegl_create_flatten_node()
and gimp_gegl_apply_flatten(), which controld the color space --
linear or perceptual RGB -- used for the operation (instead of
hardcoding it to linear).

When removing a layer's alpha channel, use the layer's composite
space for the flattening.  When flattening an image, use the bottom
layer's composite space.  Keep using linear space when creating a
channel or a mask from a drawable with alpha.