Those setters variants makes it very hard to do across project changes
to the
color type. As part of
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3544
I would like to switch from using integers to floats inside CoglColor
which this PR would simplify

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3553>