color: add from sRGB to output color transformation

This is needed when drawing anything internal directly to an output,
like the borders/decorations in a nested compositor setup. This makes
the assumption that the internal stuff starts in sRGB, which should be
safe. As borders are never blended with other content, this should also
be sufficient.

This patch is a reminder that that path exists, rather than a real
implementation. To be implemented when someone needs it.

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