We pass 2 GeglColor through the wire now. Since it is passed very early
(when sharing the configuration), I had some issues with initialization
order of GEGL, and in particular when calling gegl_init() before
gegl_config() inside _gimp_config(), I had a bunch of such criticals:

> Plugin script-fu: GLib-GObject: CRITICAL: Two different plugins tried to register 'GeglOpPlugIn-transform-core'

Anyway in the end, I store the passed colors as raw bytes and strings in
the GPConfig object, and re-construct the GeglColor last minute in
_gimp_config().