The border-drawing code for Layer icons
has been ported to GeglColor. Note that
GimpViewRenderer still uses GimpRGB for
gimp_cairo_checkerboard_create (), which
will be fixed in a larger commit.

In 9bee3bed, Border Average's return
value was set to NULL rather than a valid
GeglColor when the procedure was
created, which caused a warning on
launch. This has been fixed.