color: simplify color manager API with weston_output_color_outcome

I am going to need to add yet another output property to be set by a
color manager: HDR Static Metadata Type 1. With the old color manager
API design, I would have needed to add the fourth function pointer to be
called always in the same group as the previous three. This seemed more
convoluted than it needs to be.

Therefore collapse the three existing function pointers in the API into
just one that is resposible for setting up all three things.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>