... which return the layer's blend/composite space/mode.  However,
unlike the non-"_real" versions, these functions never return AUTO
-- instead, they return the actual space/mode that AUTO maps to for
the current layer mode.

When changing a layer's blend/composite space/mode, avoid
updating the drawable if the real space/mode didn't change (i.e.,
if changing from AUTO to the concrete value, or vice versa.)