Stop assuming that NULL represents the stock sRGB color profile. From
now on, query the stock sRGB color profile from the color manager.
This should be internal to libweston (core and the color plugins), and
users of the libweston public API should not be affected by this. They
are still allowed to set an output color profile to the stock sRGB using
NULL.
Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>