babl: add support for grayscale spaces

babl: add support for grayscale spaces

A grayscale space is just like an RGB space, but has a default set
of chromaticities for R,G,B. When ICC profiles are loaded only the TRC
is considered is considered (we could also include the whitepoint),
blackpoint tag is ignored (and should be baked into the used ICC
profile instead.)

This works with GIMP-2.10 but master of GIMP currently hangs when
trying to load a grayscale jpeg with attached grayscale ICC profile,
The if #0 on line 999 of babl/babl-icc.c needs to be turned into a
1 to enable grayscale icc profiles for further testing.