Since the space invasion commit, colors entering and leaving a
GimpColorTransform were often implicitly converted up to three times,
the code was simply not properly ported to babl formats with spaces.

Fix GimpColorTransform to only ever transform colors between the
specified src and dest profiles, ignoring the space of any babl
formats involved.

Also, always return a non-NULL transform, even if the transform could
be done by a simply gegl_buffer_copy(), this way we can make sure in
one central place that transforms are done correctly, no matter if
babl or lcms is used.

Added quite some docs and comments to make clear what happens.