libweston: add struct weston_output_color_outcome

This new struct collects all the things that a color manager needs to
set up when any colorimetry aspect of an output changes. The intention
is to make the color manager API less verbose.

In this first step, the new struct is added and replaces the fields in
weston_output.

The intention is for the following color manager API changes to
dynamically allocate this structure. Unfortunately, until that actually
happens, we need a temporary way to allocate it. That is
weston_output::colorout_, which will be removed in the next patch. This
keeps the patches more palatable for review at the cost of some
back-and-forth in code changes.

This is a pure refactoring, no functional changes.

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