color: handle image description that are not ready

This patch is for our CM&HDR protocol extension implementation.

When a client requests to create an image description, it can only start
using that after receiving the 'ready' event. The compositor may take
the time it needs to create the backing color profile for such image
description. But nothing guarantees that clients will do the right
thing.

This fixes some issues for not well behaved clients and also documents
how we handle image description that are not ready.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>